"Part of the attraction of the L.R. is, I think, due to the glimpses of a large history in the background: an attraction like that of viewing far off an unvisited island, or seeing the towers of a distant city gleaming in a sunlit mist. To go there is to destroy the magic, unless new unattainable vistas are again revealed."
"Imagination is cheap if we don't have to bother with the details."
— Daniel Dennett
One way of building background is to have the characters refer to things without explaining exactly what those things are. The slight confusion caused is balanced by the sense given of a larger world, outside the plot. If a story is extended to a long enough series (especially when there's a high turnover of writers), most of these cryptic references are explained and/or used as Canon Fodder, but often one or two will never appear in story. What such references really mean is a favorite subject for fan-clubs. The rise of the web has reduced the incidence of such things (creators get asked), though not totally eliminated them.
The effects of world-building are diminished the more references are made. Continuity Nods are hard enough to follow for story elements that have actually been depicted. The more you reference events, characters, and plots that the audience never sees on-screen, the more apparent it becomes that they are references to things that only exist in the author's imagination.
Compare to the closely-related Noodle Incident, Canon Fodder, Narrative Filigree and What Happened to the Mouse?. Unknown Character, Great Offscreen War and Cataclysm Backstory are commonly played as sub-tropes of this, as is Famous, Famous, Fictional. See Hufflepuff House for organizations with this treatment more referred to than seen. If the reference in question is actually explained later on, it becomes Foreshadowing, Chekhov's Gun, or Brick Joke. If not, it becomes a Noodle Incident. Interestingly, if you start following a Long Runner series from the middle (rather than from the start), every Continuity Nod in it effectively becomes a Cryptic Background Reference for you, so it's all just a matter of perspective, really.
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Considering Blame! is a series that prides itself in its sheer vagueness, it isn't a big surprise that this trope shows up at least once a chapter. Technology and factions are seen, mentioned and interacted with often, but in most cases, never fully explained. This adds to the already frighteningly vast feel of the 'Verse, and leaves plenty of room for Wild Mass Guessing.
Cowboy Bebop does this all the time, sometimes without even directly saying anything. It oftentimes does this with the main plot and character traits - e.g. Ein is a 'data dog', something apparently important and rare which is only vaguely explained, even in the episode where it's mentioned. This is subtly lampshaded - Jet angrily asks Spike why he's running off to kill a man from his past. Spike pointedly (even cynically) asks him how he lost his arm, causing Jet to clam up. The circumstances behind Spike's pursuit of the man and Jet's lost arm are eventually revealed.
Eureka Seven features the characters spouting a lot of Engrish phrases that won't mean anything until a good twenty episodes later. It gets pretty confusing when half of the spoken terms have no meaning to the viewer.
Its sequel, Eureka Seven AO is slightly better. There are a lot of references to events in the past, and several hints that there is something off about the Alternate Universe. They also managed to cut the introduction to meaning rate by about 50%, meaning that almost every truly cryptic and bizarre thing has been given some measure of explanation or meaning within 10 episodes, as opposed to the 20 mentioned above. Yes, if you just started watching, that means that the Alternate Universe thing will get explained very soon.
Mahou Sensei Negima! has a visual one: in This splash page from the first chapter of the manga, look closely at the center. Zazie Rainyday has claws◊. 250+ chapters later, this is still unexplained, as is everything regarding Zazie. It finally appears in the story proper in ''chapter 298''.
The Magic World is filled to the brim with this type of stuff, with people referring to all sorts of races, animals, places, and phenomena that don't happen here.
A very subtle one in Puella Magi Madoka Magica. When Mami explains why Witches are bad, she mentioned them luring people to suicide and causing fights to break out in certain places. The second type of bad influence is never seen in the anime.
It's also hinted that she knew Kyoko, and Kyoko's past - but this is actually confirmed in a Drama CD.
Several are found in in the manual(s). The official website describes three witches who never appeared in the anime, because only their familiars did. We don't even know what they look like. Then we have the revelation that Walpurgisnacht is only a nickname. It's real name is only listed as "?????".
Of course, Mami also mentions that Witches cause "natural disasters" in addition to the suicides. And Word of God about Walpurgis is that it's actually a Witch that grew stronger by combining with other witches.
Slayers has a hierarchy of gods and demons distributed over four universes. Only some of these deities are described, others are named or merely implied. Nothing is known about the demons Chaotic Blue and Death Fog, for instance, and less than that about their opponents. Fanfic authors have, naturally, expended much effort to fill the gaps.
Sora No Woto is rife with this trope. Most of the back story can be divided into two categories. There's the information that was lost forever during the Great Off Screen War, which isn't explained since none of the characters are aware of it. And then there are the things that all of them know about, which isn't explained since the characters have no reason to exposit out loud about it when speaking to each other.
Astro City uses this trope a lot, especially in the earlier volumes when the universe wasn't so fleshed out. There are constant references to heroes, villains, and incidents that the readers have not seen yet - and sometimes never see, since the story is more about how people think and live in a superhero world than about the actual exploits of the heroes. The author, Kurt Busiek, uses this trope a lot in his work - the same treatment is given to his magic-replaced-technology World War I story Arrowsmith, among others.
To the point where a reference guide for all of the bits in the first collected comic was three times the thickness of the comic itself. One panel could have two pages worth of 'This is the X from Y', especially in their museum base.
This pops up in another work by Alan Moore, Promethea. There are several references to adversaries and events from Promethea's past that are never seen or elaborated upon. The Shadow Queen however is probably the most notable since she is referred to several times without any details about her nature or ambitions revealed to the audience.
A Republic comic showed a Geonosian hive on the ice world Zaadja where several Mandalorian corpses were embedded into the walls. At the time of writing both races had an alliance with the Confederacy which would rule out any logical reason.
Writer Tristan Huw Jones would pepper his Tales of the TMNT stories with these, particularly when it came to characters.
JamesRoberts's Transformers stories tend to mention things, events, and places that are often very slowly elaborated upon or explained over time, if at all.
Deconstruction FicChallenge of the Super Friends: The End has the Legion of Doom enter another universe. Cryptic references are given constantly to this world's past and how it got to be that way, but nothing is outright stated, leaving the reader's imagination to connect the dots. The characters are just as baffled as the reader.
Blade Runner: "I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhäuser Gate. All those moments will be lost in time, like tears in rain. ... Time to die."
The Galaxy QuestShow Within a Show television series featured the Omega Thirteen, a device which was named and implied to be very important without ever explaining what it did (the series was canceled before it would be revealed). In-universe fans have been wildly speculating about it. We eventually find out that it allows very limited time travel - or at least, the one built by the Thermians does.
Technically, they based it on their own Fanon; in the universe of the show-within-a-movie, it was never revealed, though some fan speculation was along the same lines.
Pirates of the Caribbean's Jack Sparrow made several non sequitur references to his past exploits (e.g., "And then they made me their chief.", "Clearly you've never been to Singapore.") that enriched his character. The sequels tried to tie the movies together by creating plot points out of them.
When we actually do get to Singapore we never find out exactly what Jack did to offend Sao Feng, so this trope appears again.
Also, Elizabeth's speech on Black Sam's Spit: "But you're Captain Jack Sparrow. You vanished from under the eyes of seven agents of the East India Company. You sacked Nassau port without firing a shot. Are you the pirate I've read about or not?"
A mook in Ronin asks Jean Reno's former spy "Don't I recognize you?" He replies "Vienna" and shoots him. It's not relevant to the plot at all.
Upon its original release, Star Wars was a prime example of this, full of name-drops that had nothing to do with the plot but which combined to make the fictional world feel boundless and lived-in. References to the Imperial Senate, the Old Republic, 'big Corellian ships' and the spice mines of Kessel are a few examples. Scenes that were cut in the original release, such as Han's confrontation with Jabba the Hutt, also made things like Greedo's confronting Han over something he did to wrong Jabba seem bigger. Every single throwaway line has been since filled in to ridiculous levels of detail by either the prequels or the Expanded Universe.
Ironically the Expanded Universe has never introduced any ship to match Han's line about the "Imperial starcruisers" that are "big Corellian ships."
Lando: She's as safe now as she's ever likely to be. Don't worry about that. Han: You know, that's almost exactly the same thing you said back on Boordii. That botched dolfrima run - remember? You said, 'It'll be fine; don't worry about it.' Lando: Yes, but this time I mean it.
Will and Ned from Unforgiven often talked about their old gang.
J. R. R. Tolkien was a master of World Building, working on his Middle-Earth world from about WW1 until his death. The Lord of the Rings is full of lovingly crafted and referred-to details, many of which are left unexplained, whose stories first got public with the posthumous publications of the earlier stories.
One thing Tolkien knew from his studies as a linguist and English teacher is that some of the old myths recreate the Cryptic Background Reference effect entirely by accident, when the relevant poems or stories are lost — the medieval Finns probably had an explanation of what a Sampo is, for example, but it didn't survive the Middle Ages.
Then there are some things which never got elaborated on, even posthumously, like in The Hobbit when Bilbo makes reference to "the wild Were-worms in the Last Desert." Nothing remotely similar is ever even spoken of again.
Half of fun of reading Tolkien is this. Go read The Silmarillion and go back and read The Lord of the Rings. Now revel in all the references most people didn't get the first time around. That part of the song Aragorn sings in the Fellowship of the Ring about Beren and Lúthien? Now you know the whole story. Bilbo's song about Eärendil that Aragorn seemed to find so cheeky to sing in Rivendell? It was about Elrond's father (and mother) who he hasn't seen in five thousand years and probably dredged up some bad memories about the ransacking of his home when he was a child by the sons of Fëanor. The list goes on.
The Second Prophecy of Mandos, which describes what the end of the world will be like, is referenced (though not by name) in virtually all of the canonical stories of Middle-earth. However, the prophecy itself does not appear in canon - only in Tolkien's earlier drafts for The Silmarillion.
H.P. Lovecraft is a prime example. His stories make repeated, throwaway references to fictional books and locales, but there's little evidence that the man himself had any unified vision in mind. His pals also did the same, throwing out cryptic Shout Outs to Lovecraft and each other's works. Enticed, the readers wanted more, and piecing together such references is part of the fun of the Cthulhu Mythos.
Lampshaded continually in the Thursday Next series in the form of Textual Sieves. Roughly every other time they're mentioned, someone asks what they do, and are told that no one knows, since they're so sparsely described. Thursday asks Miss Havisham, and in turn Thursday5 asks Thursday how textual sieves work and the given explanation is "it's never properly explained."
There's plenty of other examples, such as the "Boojumorial" of Jurisfiction agents lost in action ("Boojumed", or deleted), the views across the wilderness to other Great Libraries for other languages, the City of Adventure that is the Well of Lost Plots, previous disasters in the BookWorld (apparently, Titus Andronicus used be "a gentle comedy of manners", but increasingly bad behaviour by the characters turned it into the "the daftest, bloodiest play in all of Shakespeare"), and items in Thursday's TravelBook that haven't yet turned out to be Chekhovs Guns, such as String TM. There's even more examples in the Outworld, which to Thursday is the real world. Genetic engineering means they've resurrected the woolly mammoth, but they don't have ducks; Britain was invaded by the Nazis during World War II and comedy musician George "When I'm Cleaning Windows" Formby led the Resistance, later becoming President-For-Life; The People's Republic of Wales; riots over art styles and literature; the weirder parts of SpecOps, and so on.
In the Mad Tea-Party scene from Alice's Adventures in Wonderland, The Mad Hatter proposes the riddle, "Why is a raven like a writing-desk?" Some time passes before Alice, the Hatter and the March Hare all admit that they can't find the answer. This did nothing to stop readers from persistently trying to find answers such as "Edgar Allan Poe wrote on both." Although Carroll himself eventually came up with the answer "Because it can produce a few notes, though they are very flat; and it is nevar put with the wrong end in front!"
Or "They both have inky quills"
Stephen Brust's Dragaera books use this in massive amounts. Paarfi's novels are supposed to be historical fiction novels within the universe, so they assume that the reader is a Dragaeran who doesn't need additional explanations. Vlad sometimes seems to make the same assumption, but other times he explains common aspects of the world for the reader's benefit. Vlad will also make vague references to his various other misadventures outside of the scope of the current story without going into detail. Sometimes he says he doesn't want to digress, and other times he's simply cryptic. Some of these do get explained in future novels. Brust intentionally includes them as possible story hooks for future novels without planning on where they'll go.
In the Sherlock Holmes stories, Watson is famous for this, often referring to other cases, such as the one involving "the giant rat of Sumatra, for which the world is not yet prepared". Some of these became stories in their own right (though usually not from Doyle's pen), but most remain unexplained.
Used masterfully well in Ulysses Silva's novel Solstice. There is exactly one incident where things are properly explained by the main character (even then, it's mostly clarification on things you've picked up). Everything else is left for the reader to figure out. And quite often, everything you thought you'd figured out turns out to be completely wrong, leading to many an Epileptic Tree until the very end.
Much like the Sherlock Holmes example, the Nero Wolfe series by Rex Stout begins on terms of false familiarity, and vaguely references past cases that are never fully explained.
Neverwhere is full of this. Particularly the Big Bad's motivation for sinking Atlantis. All we get is him shouting "THEY DESERVED IT!"
Neil Gaiman's work has tons of this, but Neverwhere and Stardust are particularly big examples.
In Michael Ende's The Neverending Story, many vague allusions are made to the further adventures of secondary characters, always accompanied by the phrase, "But that is another story, and will be told another time." Needless to say, said stories have never been told.
Actually plot-significant, and gives the book its title. In the ending, Bastian is told he can't leave until every storyline he started up is finished. But given the rate uncompleted plots have been created (several story hooks get created for every one he finishes) he'd never be done. Atreyu saves him by taking on the task on his behalf. The movies leave this out, resulting in an Artifact Title.
The Hitchhiker's Guide to the Galaxy is littered with these, with allusions to far-off planets and some of their inhabitants that are never explored in-depth. For instance, Maximegalon is apparently a planet with a very rich academic history, although it's never visited; neither is Blagulon Kappa, a world mentioned off-handedly several times but about which even less is explicitly stated. On the character side of things, Oolon Colluphid is apparently a very prestigious writer and an acquaintance of Zaphod Beeblebrox's (as of the sixth book), and some of his books have been named, but he's never personally encountered. The same goes for Eccentrica Gallumbits, the triple-breasted whore of Eroticon VI, and ex-president Yooden Vranx, who would have been part of the first story arc, but the author wound up not going there. What's really interesting is that the many of the stars and systems he mentions are real, like Sirius. This amuses people with arbitrary knowledge of stars.
There's also a bunch of almost-correct ones. There is no real-life planet called Ursa Minor Beta, but there is a star called Beta Ursae Minoris.
Used in the Old Kingdom books by Garth Nix, and not overused, either. He's mentioned in interviews that he's not really into world-building — he just makes everything up as he goes along.
Simon R Green often uses this trope in the Nightside series.
In the Codex Alera stories, there are several mentions of a group called "The Children of the Sun" who were, it seems, wiped out by the Alerans relatively recently (historically speaking) and, as a last action, did something, somewhere, for some reason and now there's the Feverthorn Jungle in the middle of the continent, that no one can enter for reasons which are unexplained. Though we get a rough idea of where it is (middle southeast of the continent according to the map in First Lord's Fury) what makes the jungle impenetrable (even to the Vord) is unexplained. The Alerans idly speculate that if they could figure out what the Children did, they might be able to turn it against the Vord.
Also All There in the Manual, as Butcher explained it on the website. They're another sentient race (of plant-people, with Woodcrafting-like powers), who are now all dead, wiped out by the Roman legions shortly after they arrived on Alera (and hence before they had developed significant furycrafting). Of all the sentient races that have appeared on Alera, only the toughest survive the competition.
Part of the charm of the early books of Chronicles of the Kencyrath is that the main character is a member of a race with ten thousand years of history, but our glimpses of this history is as through a glass darkly because the main character already knows her history and doesn't feel the need to monologue about it. The later books have filled in many of the references, but far from all of them.
The story of Princess Nell in Neal Stephenson's The Diamond Age starts out like this, and Nell and the Primer spend the next decade or so expanding the references.
George RR Martin has stated that not all locations on the map of A Song of Ice and Fire will be visited in the story. Consider the fact that no viewpoint character has yet visited (during the story, that is) the Lannister's home Casterly Rock, or the Basilisk Isles that show up on the map of (part of) the Eastern Continent.
Alien characters in Animorphs often allude to various other species or situations that have nothing to do with the plot; for example early books would sometimes list random species the Yeerks had supposedly enslaved, though practically none of them are seen except for Hork-Bajir, Taxxons and Gedds. There's also a whole subplot happening off-scene with a planet called Anati: apparently the Yeerks knew very little about it (including whether or not it was inhabited) and sent Visser One to conquer it, but she wound up failing for some unknown reason.
Following the tradition of the films, the Star Wars Expanded Universe make references of their own, some of them mentioned or expanded on by others, some of them never mentioned again. It gets downright fractal at times. Try hitting Random Page on Wookieepedia and see how far you can get before finding an article with one line of description and one or two appearances.
Often used in Discworld, with throwaway references to things like the politics of the Guild of Engravers (although that one eventually became The Truth), or the various notes on Sam Vimes's desk that he's too busy with the main story to deal with, creating the feel that Ankh-Morpork keeps running even when there isn't a story happening.
In the beginning of The Dark Tower, the third-person narrator often makes references to historical events and figures, such as the fall of Gilead and John Farson, as well as important characters in the gunslinger'spast. Many of these are clarified later in the series, further expanding the previously sparse world.
The Stormlight Archive has lots of these, references made by characters to things that happened in their pasts, references to the history of the world etc. Given that only the first book of a ten book series has been released, and that Brandon Sanderson loves to collect loads and loads of Chekhov's guns it's highly likely that a lot of them are just waiting to go off.
The Wheel of Time novels are full of references to epic historical events and heroes, and the landscape is littered with ruins and relics of bygone ages. Some of it gets expanded on and turns out to be important to the backstory, but a lot is just hinted at to give the impression that the setting is old and didn't just sit there doing nothing until the main characters arrived.
Even happens in-universe, with Birgitte and Rand (after he starts gaining access to the memories of his past life). Being ancient characters, they have witnessed events that happened THOUSANDS of years prior and have been losts to the mists of time, so nobody around them has any idea what they're talking about when they start spouting off references.
And because time in the setting is cyclical, many of the references are in fact to actual historical events in our time, but with the details garbled by the passage of the ages. Many of the oldest songs and stories will actually be surprisingly familiar to readers.
Mercedes Lackey's Heralds of Valdemar novels have a handy store of ancient history at which to hint. Some characters (Vanyel, Lavan Firestorm) have had their own books, but she claims "Windrider" and "Sun and Shadow" likely will not, since they work better as distant legends.
The Dresden Files contain a fair amount of them, such as the reasons for some wizards not showing up to a White Council meeting in Chicago including "He got real married", "Living under a polar ice cap", and "Pyramid Sitting".
In a case that ultimately ended up a subversion; in the first book Harry mentions that Santa Claus is real, and implies he's terrifying. Fast forward fourteen books, and we finally meet him in Cold Days, and get a good idea of why Harry would have been scared of him in the first book.
Sandy Mitchell's Ciaphas Cain books have loads, some of which get stories (a reference to hunting Tyranids on a hulk, now released as The Emperor's Finest), others are not (yet) fleshed out (his encounter with a Dark Eldar wytch, and time spent on a Tau world, for instance).
In the Super Mario Bros.Nintendo Adventure BookPipe Down!, Princess Toadstool mentions that in the past (around the time of her ancestors) there were things far worse and more powerful than the Koopa Kingdom, but no elaboration is offered as to what she means by that.
The Mechwarrior Dark Age novels (based on the BattleTech game world) made references to events that had occurred in the 65-year Time Skip since the last published Battletech novel. The result was a lot of terms used in general discussion that had no explanation - The Jihad, the Ruins of Gabriel, Apollyon, the Master and so forth. Since then, new Battletech fiction has begun to explain some of this.
Loads in the web-novel Domina. The names of cultures and walords are dropped without context, only to be explained a dozen chapters later. Since there's lots of Theme Naming and Shout Outs, the audience has some clues to figure out what the names refer to before they appear on screen.
With the Monster Blood Tattoo series, D. M. Cornish not only strives to make the Half-Continent feel lived in with side comments and throwaway lines, but he has so many side characters who appear and reappear within Rossamund's narrative that it leaves readers thirsty for all the potential other stories that could be told. There is, after all, a reason why the glossaries in the back of the books can be a third of the physical book.
Used fairly frequently in Herbie Brennan's The Faerie Wars Chronicles. Since most of the series is set in a fantasy realm with only two non-native characters present, references to simbala parlours, power outrages, border Redcaps, or The Reindeer King of Crippenmas are pretty commonplace. Some of these are given explanations in the glossaries, and a few end up connecting to the plots of later books, but many are left entirely unexplained.
The Gods Themselves provides no detail on an event that killed about 4 billion people: "Just about the time the Lunar colony was being established, Earth went through the Great Crisis. I don't have to tell you about that."
In the NaNoWriMo novel And Then There Were Monsters, some of the monsters mentioned in the text are explained in Father Mallern's journals. Most are not. Dire cattle, for example, and referenced a few times, but never seen, and in the end all we know is that the only similarity they have with normal cattle is that they still have four legs.
In Stargate SG-1, one of the four great races is the Furlings, who are never shown in 10 years of episodes (though a parody in episode 200 shows them as small furry creatures, looking somewhat like Ewoks). The non-appearance of the Furlings has become something of a Running Gag, since the parody starts with surprise to even meet them, and ends with the Furling homeworld being destroyed.
In addition to the character backstories seen in flashbacks, LOST has included a number of throwaway references that have captured fan imaginations, including Sawyer's "Tampa job" and Sayid's Basra incident.
In Doctor Who, all bets are off. The Doctor will routinely spout off about unexplained spatial phenomena, utterly bizarre alien cultures and references to the Last Great Time War and too many other things to even think about listing. These are often expanded on in books or audio dramas, and occasionally later in the show itself.
Stories with the Third Doctor (and occasionally others) often make mention of an unnamed "monk" or "hermit" the Doctor knew long ago. The audience finally met the monk in the Third Doctor's final story. Whether the "hermit" was the same person has been the subject of much debate among fans.
It's virtually guaranteed that whenever the Doctor meets the Master, they will talk about their childhood and various parts of their own backstories and identities, which of course will never be explained because of how important the mystery behind them (as well as the Time Lords and Gallifrey as a whole) is to both characters.
Characters other than the Doctor are known to do this, too. For example, Captain Jack Harkness of Torchwood fame. This mostly has to do with various sexual exploits, but he does mention actual bizarre past experiences. Random side-characters can do this too, usually to the Doctor or one of his companions, with the assumption that they're from the same period/place, and know what they mean.
We know about Torchwood 1 (from the Doctor Who episodes "Army of Ghosts" and "Doomsday") and Torchwood 3 (from Torchwood). But the mysterious disappearance of Torchwood 4, and why Torchwood 2 is one guy, called Archie, above a bank in Glasgow, ("A very strange man") will probably never be revealed.
Unintentionally done in the TV series adaptation of The Dresden Files, where things that are explained in the books are referenced, but are never explained in the series. It is possible they would have been, had the show not been canceled after a season.
How I Met Your Mother: There was much speculation as to the origins of the goat in Ted's apartment, which is mentioned in season three but abruptly dismissed with "oh wait, that was on my thirty-FIRST birthday". Fans had a whole season to wonder, but the actual explanation was, perhaps inevitably, a bit of a letdown.
How about the pineapple incident?
The title itself refers to How Ted Met the Mother of His Children. It took until the end of season 8 before we saw her face.
The constantly expanding Star Trek universe is replete with this one and usually two or three are generated per episode.
The other most common thing are events that occurred in the fictional gaps of time between the various eras (Approximately 100 years elapsed between Enterprise and The Original Series, 15ish years for TOS and The Wrath of Khan (The Motion Picture being close-ish to TOS' present), and 80ish for The Original Series and The Next Generation. Significant things happened during those periods that are frequently referred to, but almost never shown on screen ("What happened to the Enterprise''-B?" and the Earth-Romulan War being some of the big ones.) Expanded Universe naturally latches onto these things like crazy.
In the Star Trek: Voyager episode "The Omega Directive" it's mentioned that the titular directive was established after an accident with the Omega Particles happened to Federation scientists in 2269. The fact that 2269 is in the middle of Kirk's five year mission, and this neither references an episode of the original series, nor mentions any involvement from the Enterprise (but doesn't discount it) makes it all the more cryptic.
They Double Subverted this trope in regards to a specific alien race. Since TNG, there has been mention of an alien race called the Breen, though all that was known about them is that their homeworld is very cold. They finally show up in DS9, but in full body environmental suits that completely disguises their appearance, and nothing at all about their society is revealed. A throwaway line in DS9 suggests that they are extremely secretive by nature, so that not even the people in the shows know much about them.
 referred in a couple of episodes ("The Adversary" and "Home Front") to a race called the Tzenkethi. All we learned was that their leader was called the Autarch, and that they'd previously fought a war with the Federation during which Ben Sisko served as first officer of the USS Okinawa under then-Captain Leyton. The EU used this as a jumping-off point for several novels, including making them a major player in the Typhon Pact following the Dominion War.
Leverage has several of these, such as the named cons that we never see the team run, like the "London Spank," the "Genevan Paso Doble" and the "Apple Pie," which is a "Cherry Pie" but with lifeguards. Also, there's what Nate did at the Russian border. Word of God says that he may have technically hijacked a train, but that hasn't been mentioned on the show and likely never will be.
We've also seen the team coming back from jobs - one in Mexico where Parker picked up a lot of pinatas, and one in the Caribbean that went wrong in several different ways - without hearing much about what those jobs actually were.
Babylon 5 subverted this trope quite often, given that it was all plotted out in advance. For instance, the fates of all the previous Babylon stations seemed to be a case of simple world-building, to stress how dangerous the universe was. But then we learned what happened to them: Three destroyed, one vanished. Then the vanished one shows up again, having been Unstuck in Time. Then it gets even deeper, when it's revealed that Sinclair, The Captain in the first season, was in fact that Minbari prophet Valen, after he'd traveled back in time to provide the Minbari with the space station they desperately needed to win a war, which also helps to explain the random weirdness of Delenn becoming half-human. Other factors, like the unstable politics of Mars, also come up again in later seasons to be fleshed out.
Merlin occasionally references the Ancient Kings and the High Priestesses, who were apparently embroiled in some sort of war for supremacy before Uther came to power.
Burn Notice does this a lot by mentioning various ops the main characters have done before the start of the series.
The song "Red Barchetta," from the Rush album Moving Pictures, takes place in an unspecified dystopian future and relies heavily on this trope to paint an impression of the setting in the song's limited timeframe. An example:
My uncle has a country place that no one knows about
He says it used to be a farm before the Motor Law
And on Sundays I elude the Eyes and hop a turbine freight
To far outside the Wire, where my white-haired uncle waits.
The original short story that inspired the lyrics is a bit more descriptive of the setting, but still has examples.
Unknown Armies. Even if you manage to read through every single supplement and piece together as much as you can, there are still a lot of holes. But, since the game's major theme is a world of mystical insanity seething just below the facade of normalcy, it's generally agreed that it just wouldn't make sense if everything made sense.
The current incarnation of the Necrons in Warhammer 40,000 was inspired largely by a throwaway line by Rick Priestley about "the quiescent perils of the C'tan" which "lay beyond the Gates of Varl". References were quietly worked into the game over the years in the form of the C'tan phase sword and phase knife, until the release of the first Necron codex where the C'tan were finally revealed as the "gods" of the Necrons, indescribably ancient and evil monsters which feed on suns.
There are also the two "missing" Space Marine Primarchs and their Legions. Every mention of the twenty Primarchs lists numbers II and XI as "All records deleted", and EU works have consistently refused to give any detail about who they were or what happened to them.
The Horus Heresy series of novels and audiobooks have touched on the subject obliquely, ranging from intimations of an accident at the gestation stage, to something so shameful and terrible that the Imperium refuses to acknowledge them, even when daemon-worshiping, civilianmassacring, backstabbingpsychopathic unrepentantbastards are still listed in the records (admittedly, usually with the note "Explode planet on rumour of presence", but still). The exact details have never been revealed and are unlikely to be either, this is lampshaded by characters telling each other not to even think about discussing the details.
The closest we get is a throw-away line saying that the Space Wolves had previously been ordered to attack a Space Marine Legion.
Now that the game numbers something like 50 books, there's very little in Rifts that was mentioned in the first book that hasn't gotten a description by now. One of the biggest examples was the Republicans, which was an off-hand mention in the first book about a technological society living in the ruins of Washington, D.C. It was the subject of several unofficial Sourcebooks (called Netbooks) until they were finally described in the Expanded Edition of the original Sourcebook. There are still a few things here and there that have gotten mentioned but still not shown. The most notorious is the permanently-open Rift in Calgary, Alberta, and the monster kingdom that's developed there, as well as others like the Blood Druids of France.
This happens once a game in the Summon Night: Swordcraft Story games: random NPCs come into the scene and reference something that would make sense as part of a longer plot but which you don't know about. ("I would gladly fight to the death to follow the last orders of Master Shinrai!" ...Master who?) How much of this is the result of this actually being a spin-off series of a larger plot that never made it outside Japan isn't immediately obvious to English-speaking players.
Then there's Bison/Vega crowing about how "The Ancient One" couldn't face him. And Fei Long dedicating his art to "the master and his son" (actually a reference to Bruce Lee, Fei Long's expy source). And those three college students who inspired Sakura to take up fighting. And anything at all involving Gill. That's not even touching on the whole sordid tale of how Charlie Nash's death happened, or for that matter, where it happened, which has so many possible explanations by now, it qualifies as a Multiple Choice Past.
Undoubtedly the biggest issue involving said Sheng Long, the canon ending to the first tournament. Other than the basic fact of Ryu hitting Sagat really, really hard, we're never going to know for sure what actually happened. The most likely scenario, in fact, involves Ryu connecting with Sagat's chin (the usual one hit KO location for MMA); any blow to the chest forceful enough to put away a brawny heavyweight would've left a much larger scar than what Sagat's currently sporting.
Kingdom Hearts enjoys this quite a bit. In fact, each game features a secret trailer hinting at the plot of the next game... before anyone actually thinks up what the plot will actually be. To this end they've been infamously vague.
Super Robot Wars lives on this trope - Throwaway references became major plot elements in later games - such as Lemon's last name, and referring to a 'Beowulf' who piloted a 'Gespenst Mk. III' (Alt Eisen) - obscure references to the previous game, where the protagonist's theme called was 'Steel Beowulf' and his unit was revealed to have been a modified Gespenst Mk. I considered for mass production. And many, many more.
Mischief Makers treats all the characters as already established, and new characters are often brought in with the assumption that they've had encounters with main character Marina in the past.
Cave Story. There are references to shared histories between characters, and to a war ten years ago with armies of robots from "the surface" attacking the Island, and to three bearers of the Demon Crown prior to the Doctor. There's just enough information to construct a vague timeline for the backstory, but it's obvious that there's more to the history than what we're told. When asked for further information about the game's world, the writer has said that he himself doesn't know.
Sid Meier’s Alpha Centauri includes many fictional quotes from the game's characters and other people in the game's world. These as well as a handful of other game elements are the only inside look we have into what life on Planet is really like (unless you read the novelizations, anyway). Many details are left to the imagination, such as, "What exactly is 'nerve stapling', anyway?" Somehow, it works.
In Tsukihime, during the final encounter with Nrvnqsr, he and Arcueid have a conversation on things that you don't learn until much later in the game, or in supplemental materials. This is intensified by the use of code-like terms, such as referring to Roa as the "Serpent of Akasha".
Team Fortress 2 has this to an extent. It began with the Excuse Plot of two rival corporations hiring mercenaries to fight over seemingly trivial objectives. Each mercenary has a distinct and interesting personality, but virtually no Back Story is given and they didn't even have any real names. Since the release, more of the history leading up to the game has been revealed, and additional bits about some of the classes have come forward.
Potentially, the increase in backstory has only increased this trope. Why does the Announcer control both sides, pitting against each other for no apparent reason? What are her connections to Saxton Hale and the Redmond and Blutarch families? Why is friendship such an alien concept to her? What will the Engie do with all that secret deposits of Australium? How did the Soldier get a magician as a roommate? (All this only appears in the addition material. If you're content to run around reducing other players to bloody chunks it won't bother you. After all, reducing each other to bloody chunks is the point of the game.)
The Half-Life series is almost entirely built out of this trope. What were the various departments at Black Mesa researching? What do those vast Combine machines actually do? How did those corpses end up where they are? Who was the Rat Man? Who was Lazlo? The vast majority of the story is told by implication only.
BlazBlue has six legendary heroes. Hakumen is playable, Jubei and Valkenhayn Hellsing show up as NPCs and the rest are shown only as silhouettes until ContinuumShift. It adds Terumi Yuuki to the playable list, adds Platinum (one of the three souls in her body being that of Six Heroes member Trinity Glassfield) to the NPC list and reveals that Nine is dead. Valkenhayn and Platinum later became playable through DLC and expansions, and the silent villain Phantom is heavily implied to be a brainwashed Nine.
Fable I and Fable II gives us all kinds of elaborate references to places you never go, ancient tribes and cults, and legendary heroes, none of which are ever seen.
The driving force behind The King of Fighters, Orochi, qualifies. The only things we know for certain are 1. it involves a horribly evil destructive power, 2. the clan is absolutely fanatical and will stop at nothing to raise their god, 3. the Kusanagi, Yagami, and Kagura clans were the ancestral foes of Orochi, 4. but the Yagami betrayed the alliance, causing their flames to turn purple. Everything else is a confused mishmash... some artifacts we never actually see (until Ash Crimson starts pilfering the current clan members of their powers in the 2003-XIII arc), "maidens" who may or may not have been slaughtered, Kyo getting preferential treatment causing Iori to go all emo or something, a bunch of sealing and unsealing attempts, "battle energy", earth worship and "returning all to nothing", Rugal of all people chosen as a guinea pig, self-destruction, betrayals, counter-betrayals, etc., etc. Worst of all are the numerous plot points and outright sequel hooks that are flat-out dropped, such as a heavily-implied rift between Rose and Adelheid at the end of XI.
Those from the Past's goal to obtain the power of Orochi by breaking its seal (yet again) comes across as this, as it's never clearly spelled out during the Tales of Ash what their leader Saiki wants to do with it. Not even in XIII, the climax of the saga wherein said leader makes a formal appearance and all of his scheming comes to fruition. Side materials reveal that the group is more or less the Western European equivalent of the Orochi clan and Saiki needs Orochi to power an artifact known as the Gate (which does appear in XIII) so he can travel back in time and change the flow of history so as to prevent the cult from suffering a horrible loss to their human adversaries (presumably Elisabeth's ancestors, thus explaining her personal vendetta against them and the repeated, equally cryptic mentions of her and Ash's "mission"). Sadly, little of this is explicitly mentioned in the game proper, and Saiki's shady, self-serving nature (complete with the implication that said explanation doesn't hold water) opens up grounds for further discussion.
If there's one plot point in KOF that adheres to this concept more than anything else, it is—without a doubt—the Dragon Spirit. That subplot first reared its head back in KOF '99, with Kensou and Bao shown to be vessels for an otherworldly power coveted by the mysterious Ron (former leader of the Hizoku clan of assassins and a former member of NESTS, the villainous cartel of which KOF's third Story Arc centers on). Fast forward to XI and XIII and all we know is that Kensou has apparently mastered its powers (much to Ron's approval) and Ron has unspecified ties to Those from the Past (as Saiki alludes to Ron having warned him of Kensou in their pre-boss fight dialogue). We know nothing of the Dragon Spirit's origins or why Ron has his sights on it, although it can be inferred that this has something to do with Ron's betrayal of his clan. This is mainly an effect of the plotline mostly being put on hold in lieu of Those from the Past's machinations; with several Hizoku members being introduced to the cast since 2000 (including Ron's son Duo Lon, a semi-important supporting character from 2003 onward) and the Tales of Ash having come to a close with XIII, it's possible the next arc will finally revisit these story elements and elevate the ever-elusive Ron to main villain status.
Silent Hill is another prime Canon Fodder series, as this trope tends to be the only source of available information. With one exception (Walter Sullivan, the main enemy in Silent Hill 4, was originally mentioned in a newspaper article in Silent Hill 2) none of them are explored or elaborated upon, and in the case of some this is probably for the best.
The Saboteur: What is the trouble that made Sean unable to return to Ireland? Who are the enemies he made there? What did Sean's father do there exactly? Where did Sean learn to use explosives? What was Sean doing in Budapest when he met Skylar?
It's possible that the time period immediately before the literal "Troubles" in Ireland is referenced, and this would also explain how he learned to use explosives.
Final Fantasy VIII does it occasionally, most notably with the sorceresses. For example, "Great Hyne" is mentioned as their progenitor and source of their powers, but you never learn who exactly that was, save for some legends told by NPC you're likely to miss.
The Elder Scrolls has a lot of background material that is only hinted at in the games.
Golden Sun games have a lot of this kind of information that you can gather from the various NPCs or furniture in the various towns, regarding the political and economic situations of your surroundings, gossip about local leaders, what the cuisine and culture are like in each place (in some places extending to religious beliefs), optional content you can explore, and once in a while some obscure lore tangentially related to the plot.
Dark Dawn takes this a whole step further with the major characters themselves discussing some stuff in relatively major cutscenes that isn't at all involved in the plot of the game, like the three races of the Precursors, the modern geography and political unrest around Morgal and Bilibin, and Kraden's messenger pigeons. Fans were annoyed that this cut into plot and character development, which were less consistent than in the previous two games.
Dragon Age: Origins has a lot of background material that isn't wholly relevant to the games' plot. Moreso in the sequel since the game takes place on a more personal level. Some particularly interestingbits of background are that the Primeval Thaig was apparently part of an ancient dwarven civilization predating the Deep Roads, which had temples and worshiped gods (and that the Primeval Thaig was ruled by a "dwarf so foul the Stone rejected him" and covered in evil red lyrium that drives people insane), that at least one Eluvian leads to "another place - beyond this world, beyond the Fade", that Flemeth is, at least according to Morrigan, something other than merely a mage or an abomination, and that the qunari must have arrived in Thedas (the continent where the games are set) from somewhere because they only showed up three hundred years ago, but no one knows where they came from.
More subtly, there is the whole Codex business: in theory, it should offer exhaustive explanations and backstories of in-game events, items, and characters. However, in practice, one cryptic reference explanation contains three more references that don't get explained.
The Warden can do this during Witch Hunt, only vaguely hinting to their new companions their precise relationship and reasons for searching for the Witch of the Wilds. Particularly noticable if Morrigan was romanced, where Ariane eventually comes to realise that the Wardens cryptic statement that "She has my child" didn't mean that Morrigan had kidnapped their child as she'd assumed, but actually because Morrigan was the mother of their child. Furthermore, she later figures out the nature of their relationship, asking the Warden if they are aware they subconsciously play with the ring on their finger when they think no-one is looking. After the Warden explains it's part of a pair shared with Morrigan, Ariane is genuinely amused when they fervently deny that this means they're married.
Xenogears. A lot of the backstory involving dead civilizations and their predecessors from space but you only know (sparse) details from supplementary materials. Squaresoft was hoping to make sequels based on this information.
Girl Genius does this a lot, helping to give the sense that it's an alternate history defined by the presence of mad scientists. Among the more notable are references to what things were like before Baron Wulfenbach took over Europe, what the places ruled by less pleasant Sparks are like, and the fact that there are multiple popes.
Occasionally they'll also drop and/or hide names we're familiar with in places we can spot them, showing how those individuals are different in this version of history (the most prominent one is actually reasonably well known, but he's addressed by his surname where we the reader are typically familiar with his given name alone). It's Rembrandt van Rijn.
The most recent story arc in The Inexplicable Adventures of Bob! has involved a lot of clarification of throwaway details like this from earlier in the series. We've finally seen Butane the planet of dragons; we've gotten a minimally technobabblish explanation of what borfomite actually does; we've seen some court intrigue in the Nemesite Empire; Fructose Riboflavin is finally looking competent enough to explain how he got his terrifying reputation; etc.
Dresden Codak engages in this from time to time. We never learn anything more about Reverse Moses, other than that he once parted the city to escape Aqua Pharoah.
All over the place in the Whateley Universe, because the authors have a huge bible they're working from. So there are references to superheroes and super-teams we haven't met yet, and supervillains who are 'A-level threats' according to an international scale we haven't had explained either, and also tons of references to real-world things to show how close that universe it to ours.
Some of these are All There in the Manual. A B-List is world-threatening, but your average supergroup can still maybe win. Maybe. If they're lucky. An A-list, you have to call in EVERYONE. An A-list is the kinda guy you have for a Crisis Crossover.
Also present in The Descendants, with characters mentioning minor villains they've defeated, superheroes in other cities, and seemingly pivotal moments in history that haven't even been explained in flashback. Whether or not they're just building up a Chekhov's Armoury, though, remains to be seen.
Doctor Horrible's Sing-Along Blog makes mention to other superheroes/villains outside the main characters, such as Bait and Switch, Johnny Snow, and Hourglass, although some of them do get actual "screentime" in the supplemental comics.
Limyaaelrecommends that if you're going to use these for worldbuilding, use a lot of them, so the audience gets used to them as worldbuilding and doesn't expect them all to be plot points... and the ones that are plot points have the element of surprise.
Megatron in Beast Machines has a scar over one eye which he didn't have at the end of Beast Wars. Where did it come from? We don't know.
The three-part pilot episode of Justice League shows the Big Seven superheroes working together as a team for the first time. This was Green Lantern John Stewart's and Hawkgirl's first appearance in the DCAU — and the established superheroes knew exactly who they were without any introductions, having apparently heard of their off-screen exploits or worked alongside them at some not-shown point in the past.
Similarly, a lot of the superheroes and supervillains in Justice League just show up with their origins unexplained (and often never explained) as well; The Flash, Red Tornado, Vixen, Gypsy, Star Sapphire, Copperhead, Black Manta, and Doctor Polaris are just a few examples.
Another big DC Animated Universe example - The Near Apocalypse of 2009, mentioned in Batman Beyond and Justice League Unlimited's Epilogue. Nothing is known about it except it was the last battle between the first Batman and Ra's al Ghul where Talia finally betrayed him for good. No other details are known about it, and so it has become a popular subject whenever fans demand more closure to the DCAU, although Word of God says they never really planned to cover the event in any of the animated shows, and at this point it does not seem like they ever will.
Futurama often makes fleeting references to events that happened in the thousand years between 1999 and 2999, such as Conan O'Brien losing his legs in the War of 2012, most videotapes being destroyed in 2443 during the Second Coming, and the Sith War of 2865.
Adventure Time has many references to a Great Mushroom War that happened about a millenia ago and that the world may have been like ours before this war.
For instance, one of the more daunting villains and terror incarnate, The Lich, has his home base in an abandoned derelict metropolitan subway station that survived the decay of millenia. Why the preference for a cityscape instead of the more typical and practical evil overlord lairs?
Buildings and leftover architecture litter the landscape, but are quickly panned over as scenery all the time. A Freeze-Frame Bonus view of the Earth from space reveals that it's missing about 1/4 its mass, with a massive hole in the side.
An ordinary movie with the standard disclaimer for home videos once appeared. Jake noted that it was from "before the Mushroom Wars". As in mushroom clouds?
There are also more typical references, such as Billy the Hero's past exploits (He apparently managed to defeat the Lich, mentioned above, with Good Old Fisticuffs. Meaning, he somehow manage to punch him hard enough and for long enough against a tree that he became sealed in amber. That's the impression of events, anyway.) There was also a Rainicorn-Dog war and a group of Fish People who have colonized what may be, but is never confirmed as, a human fallout shelter. Etc, etc.
In one of American Dad!'s Christmas specials, taking place on the post-Rapture Earth, inhabited by demons.
Stan: The open road is too dangerous. Jesus: What if we go through Sector 16? Stan: Sector 16? The perfect man just proposed the perfect way to die. Jesus: Fine! Then what about Sector 35? Stan: Sector 35? (scoffs) Sector 35 makes Sector 16 look like Sector 48!
Star Wars: The Clone Wars had the Death Watch start working for the Confederacy in one episode and planning to kill their leader Count Dooku during their next appearance, where they were stranded on a snow world. Why this was happening was never explained.
ThunderCats (2011) has the Cats searching for three stones of power, in addition to the Eye of Thundera. Where these come from no one knows. Also, the planet they live on is called Third Earth, and unlike in the original series, this one is heavily implied to not be our Earth, which means there must be a second earth. It's also implied that technology once existed in abundance even before Mumm-Ra's pyramid crashed there, which means it could be After the End.