Sometimes, it's considered a mark of good World Building in the Science Fiction genre to develop an alien culture like you would any aspect of the story. To talk about their history, culture, motivations and character.
Other times, writers find it best to leave these questions for the ages.
The aliens are simply there. Any attempts at communication are met with silence. Their origins, motives, and plans are all mysterious. Nothing is known about their society and culture. Their actions come across as bizarre, incomprehensible, and frequently terrifying. Your best bet is to stay far, far away from them and not do anything to attract their attention, because the consequences of even being in their way are frequently horrific, let alone when they have noticed you.
Often the aliens in question are Starfish Aliens and may speak a Starfish Language. Given most are malignant invaders, many times they're Planet Looters. While they are not required to be Eldritch Abominations, per se, they usually wind up in this territory.
Please don't confuse this trope with Inscrutable Oriental.
- The JAM from Sentou Yousei Yukikaze. It has been 33 years since their attempted invasion of Earth. In that time, the Faery Air Force has been attempting to figure out what they are and why they invaded. After over 3 decades of fighting, they have learned absolutely nothing about them. Everything about the JAM that might be remotely accurate is all pure speculation. It also does not help that they may or may not even have physical bodies, as the only thing the FAF has seen of them are their aircraft and the makeshift "runways" they use to launch from.
- The Zai from Girly Air Force, which cribs heavily from the JAM example above from Yukikaze.
- While the audience may have obtained a lot of information about the Angels of Neon Genesis Evangelion in between the show's initial release and the current day, most of it is strictly All There in the Manual territory and a manual that is very hard to get at that. In-Universe, most of the characters have no damn idea of what the Angels are or what the heck they plan and the ones that do have a clue are both absurdly cryptic about it and it's hinted that they only have just enough information to make their schemes and that is about it.
- The Image Comics series Trees follows the many misadventures of people who are part of an Earth that knows that aliens exist, because multiple gigantic black "posts" landed around the world (the Title Drop lies on the fact that humanity has had as much luck contacting the aliens that sent or are inside said posts as they would have asking a tree to talk, and so "like trees" the posts watch the world unfold).
- The true motive and appearance of the aliens in Annihilation (2018) remains mysterious.
- Independence Day: The aliens just show up and blow up cities, the only time they communicate with humans is when one is taken prisoner and takes telepathic control of a scientist in order to demand its release.
Peace? No peace.
- More is revealed in the sequel, like the fact that they're a Hive Mind and like to extract planetary cores. There's also a lot more than a single fleet.
- The aliens in Battle: Los Angeles don't make any attempt at contact before they crash in and start killing people.
- Brightburn: The aliens that Brandon belongs to are this, in stark contrast to Jor-El and his fellow Kryptonians.
- The Engineers in Prometheus seeded life on Earth billions of years ago, guided early humanity and left maps to their nearest outpost, then two thousand years ago decided to send a ship armed with bio-weapons to destroy everyone, the only thing that stopped them being that the weapons broke out. One of them escaped and made it to a stasis pod, to be awakened 2000 years later by human explorers. David had a grasp on their language and even spoke a few words to it, but its first act upon waking up was to start killing people and launch the ship.
- It's even stranger when you consider that this impossibly advanced race is genetically identical to humanity. They look like humans but are utterly alien.
- Arrival essentially takes this type of alien and has the protagonist attempt to communicate with them despite their inscrutability. They appear without warning in vessels shaped like giant lenses, with no obvious technology inside or outside. They look like giant seven-fingered hands, and have nothing resembling a face. They let humans in at scheduled times, apparently just to tromp around in front of them making incomprehensible noises, sometimes making jagged black circles out of fog. When Louise becomes relatively fluent in their language, she gains a sort of Mental Time Travel ability. She doesn't know how this works; it's Hand Waved as having something to do with the non-linear language structure. The aliens tell her that humanity will need this inexplicable time-warping language to help them with something in 3000 years, then they disappear; that's as close as we get to knowing anything substantive about them.
- The alien probe in Star Trek IV: The Voyage Home approaches Earth emitting an extremely powerful signal that resembles the calls of humpback whales, and all other attempts to communicate with it fail completely. It leaves when it receives an answer from a pair of time-shifted whales, and nobody ever figures out what it wanted to know, where it came from, or who built it.
- As with many Alien Invasion tropes, the Trope Codifier is probably The War of the Worlds. The Martians make no attempt to communicate and the humans can only speculate on their motives.
- The Ender series has a word for such a species: varelse. More than one of the species placed in this category were later reclassified as the understanding between them and humans improved.
- The entire planet in Solaris is an Eldritch Abomination, the workings of which are fundamentally incomprehensible; what it does is frequently hard to describe, and why it does these things is wholly beyond anyone's grasp — the scientists studying it can't even figure out whether or not it's trying to communicate with them. At least a third of the book consists entirely of the protagonist recounting various scientific papers hypothesizing about how and why Solaris works — his point being that they are all nothing but blind guesses with equal chances of being true or untrue, because none of the available facts permit Earthlings to come to any rational understanding of it.
- The Ark Megaforms in Space Mowgli, though they do attempt to communicate with humans via the title character.
- The Wanderers in the Noon Universe novels: humanity finds traces of their presence all over the galaxy but has never encountered one in person.
- Variation in Blindsight. The alien ship does make contact but the crew quickly figure out that they're talking to a "Chinese room". They spend a lot of the novel trying to figure out whether the giant starfish-like creatures patrolling the ship are sentient or just drones of some kind as they make no attempt at communicating. The truth is far more sinister: the aliens are not sentient, self-awareness is not required to build starships, sentience is an anomaly unique to H. sapiens sapiens and will be corrected eventually.
- John Wyndham's The Kraken Wakes (retitled Out of the Deeps in the US): A group of aliens invades the Earth's oceans. They never contact the human race in any way and the two sides engage in a war without either side ever seeing the other face-to-face.
- Robert A. Heinlein's short story "Goldfish Bowl". Unknown creatures (it's not clear if they're from Earth or aliens) suck a huge pillar of seawater into a cloud and then return it to the ocean. They also send out fireballs that kidnap people. They're never seen by humans and don't communicate with humanity.
- Stewart Cowley's Spacecraft: 2000 to 2100 A.D.. The City Ships of Alpha are huge cities that float from one pillar to another at irregular intervals. They have existed as far back as the Alpha Centaurians' history goes. All attempts by the Alpha Centaurians to communicate with their occupants have failed, and all attempts to enter the cities are prevented by a protective force field.
- The contagion embedded in the deep space probe in The Andromeda Strain is a crystal-based organism that kills Earth creatures by reorganizing the minerals in their bodies to fit the crystalline template. It turns out, the crystals have predefined stop points, leading to the hypothesis that the organism was created by an extraterrestrial intelligence as a data storage medium.
- John Varley's Eight Worlds series involves a Transhuman future where humanity has colonised much of the Solar System and then one day aliens came and evicted humans from Earth, with no (visible) rhyme, reason, nor response when contact was attempted.
- In Christopher Nuttall's Ark Royal the aliens ignore all calls for communication and simply dice humanity's ships to pieces. Though late in book 2 a faction of the aliens attempts to make contact, before being destroyed by the war faction.
- In Edmond Barrett's Nameless War the Nameless make no attempts at communication, expeditions after the Mississippi Incident discover their handiwork but not the Nameless themselves. In Last Charge the Nameless do offer terms of surrender - humanity must stop breeding and go extinct peacefully.
- Janusz Zajdel had these in Limes Inferior (very inscrutable, we can only speculate why they invaded, and most humans don't even know there's been an invasion in the first place), as well as Wyjście z cienia (we get a chapter from their point of view).
- The Presger in Ancillary Justice exist mainly as an unknowable, outside force that has put a dampener on The Radch by proclaiming humans a 'significant' species that are worth not being disassembled in an amusing fashion. From what little we learn from one of their Artificial Human 'translators', their way of thinking is completely alien to ours and they don't seem to understand individuality or personhood, or at least not in a way translatable to us.
- Eden Green follows a rationalist young woman attempting to learn the origins of the alien needle monsters invading her city, including the horrifying symbiote they sometimes transmit to humans. After crossing over to their world, she discovers a mountain stronghold, abandoned for centuries (or more), and gains zero useful information about the monsters, symbiote, or ancient alien civilization.
- Nancy Kress's short-story "Savior" starts Twenty Minutes In The Future, when an alien ship lands in Minnesota and...just sits there, completely inert and quiet, protected by an impenetrable force field, for the next three centuries. Civilization experiences a minor collapse, and then bounces back better than ever, and still the ship just sits. It isn't until researchers in China create the first quantum-based artificial intelligence that the ship suddenly takes action and steals the AI, apparently to protect it. But we still have no idea how to communicate with it or any real idea of what its motivations might be.
- In The Three-Body Problem, this is the key issue that makes the galaxy such a terrible place: because it's so hard to understand other species, it's essentially impossible to establish trust, civilisations have to assume everything Out There is hostile, and as such, the objectively correct response to first contact is to schedule extermination.
- Star Trek
- In the Star Trek: Enterprise episode "Silent Enemy," the Enterprise encountered a species that answered hails but did not speak, and soon turned inexplicably hostile.Expanded Universe
- Star Trek: Deep Space Ninenote has the Breen, a race of Humanoid Aliens who always go around in full-body, face-hiding spacesuits and speak only in a deep electronic warble. There's several (often mutually contradictory) pieces of exposition about their physiology, but apart from them allying with the Dominion in exchange for taking possession of Earth there's little known for certain about them. The contradictory information is acknowledged on-screen as intentional: allegedly their space suits are cooling units because they would die at anything above sub-freezing temperatures, but Weyoun remarks that he's heard that their homeworld is actually temperate, so the "cooling suits" explanation must be a lie to cover...something else. Expanded Universe
- Star Trek: Voyager:
- One episode had them rescue an alien that was so bizarre they had to start from scratch on trying to understand it. Its biology was such that the medical computers, including the Doctor, couldn't make sense of it, and its language was beyond the universal translators capacity to decode.
- In another episode, Voyager itself played this role. They were in orbit around an unknown planet. Unbeknownst to them at first, time passed much faster for the inhabitants of that planet, so from their point of view Voyager had been in their sky for centuries, and was a complete mystery to them.
- In "Twisted", an Unknown Phenomenon turns Voyager into a Mobile Maze of rooms and decks all jumbled up. The problem sorts itself out and afterwards the crew discover their computers have been scanned and a whole lot of new information downloaded. Janeway believes that the whole affair was an attempt at communication, but the nature and motivations of whoever did this remain a mystery.
- The Stargate SG-1 episode "Grace" had the Prometheus encounter a ship which refused to respond to hails and opened fire on them, chased them into a Negative Space Wedgie which trapped them both, and abducted all of the crew save Samantha Carter. Even after Carter manages to figure a way to get everybody out (using a hyperspace "bubble"; It Makes Sense in Context), the alien ship doesn't respond to her when she hails it to demand the return of the crew in exchange for help; it just teleports the crew back.
- Stargate Universe had the crew aboard the Destiny chased by a Starfish Aliens species (dubbed the Nakai by executive producer Joseph Mallozi) that only transmitted the word "surrender" as a constant enemy, and the crew ends up finding little (if any) information about them during the course of the series.
- The Vorlons from Babylon 5. While they can be spoken to, it's debatable whether they can be communicated with.
Sinclair: Yes we have files on them, very large files. There's nothing in them, of course.
- The Midnight Entity in Doctor Who was so mysterious that not even the Doctor knew what it was, where it came from, or what its goals were.
- The Mothership Zeta expansion of Fallout 3 features the player character fighting his way through an alien space ship. We hear a variety of accounts of (horrifying) experiences with the aliens from a number of people, but the human characters can only guess at the aliens' motives for the atrocities they commit.
- The Khaak, a species with point-to-point jumpdrive technology that invaded the Community of Planets in X2: The Threat, and was finally defeated midway through X3: Terran Conflict. No attempts to communicate with them have gotten a response, prompting speculation in-universe that they're just too mentally alien. All the Community of Planets races have learned is that they are attracted to nividium and that they seem to be Bee People rather than individuals (the latter from having cracked their video communication codecs).
- The Outsiders from the backstory. They tried to invade this universe from another one some millions of years ago. The Ancients tried every method of communication they could think of, but never got a response. Finally, the Ancients blew up an Outsider scout/colony ship, and that seemed to convince them to back off, but pretty much nothing is known about these beings even now.
- The Shivans in Freespace never communicate with anyone. They reveal nothing of their culture, or their mindset, or anything about themselves. Their only method of interaction is by destruction. The only things we learn of them are through ruins and left-behind information by the Precursors that had previously fought them (and lost), and consists entirely of their wild guesses. The endgame of the second game has the Shivans perform some very strange actions that are completely unexplained (and also make clear that they could have wiped out the humans and vasudans way before, but had chosen not to). No explanations are forthcoming either because the games were cancelled at this point.
- In the intro video to Sword of the Stars First Contact with the Hivers is described thus: "They came in silence, no statement of intention, no declaration of war. Without even a demand for our surrender." Though, of course, other factions of the Hivers proved more talkative, as well as all the other playable species out there.
- World of Warcraft has the Naaru. They're basically portrayed as space angel windchimes whose only known motivation is stopping the Burning Legion (who are demons/evil aliens). They also have absurdly high tech spaceships which for some reason seem to be built to be run by creatures with hands, like the draenei. Of course, in the Exodar's case, O'ros simply could have been piloting the ship psychically. He either lost concentration or crashed on Azeroth on purpose. The Naaru are known for doing harmful things to themselves to further their goal of stopping the Legion. Basically nothing else is known about them and they cannot/will not communicate in any way beyond psychic image projection.
- Arcen seems to be fond of this on the whole. The three alien races from the AI War: Fleet Command expansions are very non-human in their wants and priorities, though their view of humanity ranges from being friendly to antagonistic depending on the racial group.
- This carries over to Stars Beyond Reach, where Spire prove to be even weirder when they aren't intergalactic refugees. By contrast (AI) Zenith are clearly making an effort to integrate with other races but they aren't good at it.note
- The Last Federation's Obscura put the rest to shame though; they're a particulate based form of life with which communication is impossible.
- The Varelsi from Battleborn are a legion of extra-dimensional cosmic horrors who have for thousands of years been dragging stars back to their dark dimension, essentially consuming them all for some unknown reason. Very little is understood about these eldritch abominations and less their actual motives especially since violence seems to be the only language they communicate with to those whose stars they consume. There seemingly is no way to communicate with these beings yet Rendain was somehow able to broker an alliance with them in order to save his people the Jennerit from the Varelsi's onslaught as he came to the conclusion that the complete heat death and darkening of the universe was inevitable.
- Muv-Luv: The BETA are completely inscrutable. Numerous attempts to decipher their language were made, but ended in failure when humanity was unable to determine if the BETA even have a language. This eventually culminated in using limited telepathy in an attempt to get anything at all out of them... which also failed, as the impressions and thoughts they got from the BETA were too out-of-context and vague to be understandable. All humanity managed to figure out after forty years of research is that the BETA do not consider humans to be living beings.
- There is one remote civilization in Meteos of which the only known aspect is what their people look like. They have an Astonishingly Appropriate Appearance, having small round bodies with huge floating hook-shaped heads such that they look like question marks. The size of their planet, population, and people are completely unknown, and as such, they're given the fitting name of simply "Unknown." This civilization is so mysterious that it's the only planet in the galaxy with confirmed intelligent life that Planet Meteo does not target for annihilation—even Planet Meteo knows too little about Planet Unknown to attack it.
- None of the aliens in Anna Galactic seem particularly talkative aside from the Hishla, who speaks through Anna while slowly consuming her mind.