The structures making up the Dark Ones' home are massive Soul Jars housing the souls of the deadNotice how virtually no ghosts were present on the surface in Metro 2033 when compared to Last Light? This was because the Dark Ones had somehow collected many of them into the massive, coral-like structures that comprise their home, in an attempt to grant them peace. But when Artyom destroyed their home, many of the ghosts drifted back toward their former resting places while others were trapped around the Red Square.
The mushrooms the Metro inhabitants are growing are really magic shroomsThe Metro has just a few hundred inhabitants (because it couldn't sustain more due to lack of space and light to grow enough crops), all the adventures in Metro are just the hallucinations of Artyom. All Metro inhabitants sit around all day in an empty, completely non-mystical metro station, they sometimes go into the (completely non monster-infested) tunnels to foster their mushrooms when they are more sober. Not all of the mushrooms are "magic", of course, most are simply champinions. Artyom is just having a bad trip. Oh, and the pigs don't exist either. Feeding farm animals when supplies are low anyway? Not a chance.
- It's hardly unreasonable to assume that the stations can support pigs, as they can subsist on things that humans would consider inedible. Moreover, the Metro's inhabitants are clearing growing crops other than mushrooms, using either electrical light or in some cases, natural light from the surface.
The Moscow Metro is not the last outpost of mankind.It seems odd that the equally deep-sunk Metroes of St Petersburg, Nizhny Novgorod, and Gorky would not also see microcosms of the Moscow conflict. Similarly, the entire world may not be devastated - the vast Russian continuance-of-government stations in Siberia might still be in operation, trying to build enough of a society to ultimately reclaim the shattered Western Russia - the ringing phones in D6 would support this theory.
- Metro: Last Light mentions that for a year or so after the nukes hit, the Metro was held together with the hope of contacting the President who was believed to be in a bunker in the Urals. However, all attempts were unsuccessful and order gradually broke down. Given what NATO or possibly the Chinese may have hit the Kremlin with, it is possible that similar weapons were deployed in other government bunkers.
- Furthermore, it's unlikely that both Russia was the only place affected by the nuclear holocaust and that the Moscow Metro is the only human-inhabited place left in the world. Realistically, Russia would have retaliated in kind, so the Western powers were probably nuked to some degree as well. There could totally be people living in similar conditions to those in the Moscow Metro in other countries; for example, New York's subway system or the London Underground.
- It may not be exactly the same canon, but in the book series, the St. Petersburg metro is also occupied by survivors, and there is a book set there, with its own protagonist. Also, at the ranger base at the church in Last Light, IIRC one of the NPCs mentions having finally made successful radio contact with St. Petersburg, although one of the other NPCs doesn't believe him.
The "Enlightened" ending is actually Artyom getting Mind Raped by the Dark Ones.At the end of the day, we really don't see any evidence that the Dark Ones desire peace and are any more than monsters apart from the visions, which are sent by the Dark Ones. That hardly makes those visions an objective viewpoint. Also worth noting that in the final visions, they attempt to make Artyom jump off the tower, and only stop their attacks and explicitly sue for peace when Artyom fights off the vision and shoots the Dark One giving it. He has no real reason to believe that they deserve to be spared. The only Dark Ones seen in game are those forcing visions into his head, and it is an unprovoked attack by Dark Ones on Exhibition Station that triggers Artyom's mission in the first place.
Metro 2033/Last Light takes place in the same timeline as The Last of Us.As was pointed out in the WMG page for the latter game, they both begin with a cataclysm of some sort in 2013, and then move forward 20 years to the "present"— the year 2033. As for how the plots are connected, the story about the infection originating in South American crops was a cover— it was actually deliberately engineered by the US military, for use as a first strike weapon against Russia. However, it leaked out somehow and began infecting Americans, culminating in fungus-crazed military personnel launching missiles at Russia, to which the Russians responded in kind. Furthermore, the mystery superweapon used against the Kremlin was actually a payload of cordyceps spores that infected and killed everyone within the radius of Red Square. It was subsequently mutated by exposure to radiation, becoming the sentient Biomass that dwelled in the Kremlin basement and lured Stalkers to their doom with its newly-evolved psychic abilities.
- If I may offer an alternative approach, the war did break out due to mundane reasons but during the war some of the American bioweaponry got accidentally released upon American soil and gradually spread. It could also explain why we don't see much of a NATO remnant/expedition in the Metro games even though the interactive map shown on the website suggests not the whole of US was hit by nukes: they are too busy keeping order in their own backyard to bother checking to see what happened to Russia, especially if satellites show no sign on the surface.
The mutant horde that attacked the Rangers was sent by the Dark Ones.Simple enough, really. The Dark Ones are established as having psychic powers. It's not too much of a stretch that they could use those powers to goad all those watchers and demons to attack right as Artyom, Miller, and the others arrived outside their destination. They're clearly shown trying to stop Artyom from launching the missiles when he reaches the top of the radio tower, but the initial attack was to try and stop them from getting that far to begin with. For the most part, it worked, since it killed/scattered most of the Rangers, but it was only after Artyom got really far up the tower, far away from where any other mutant to get him, that they intervened personally. The chips were down at that point, it was their last ditch effort to save themselves, but the horde was their opening gambit rather than pure chance.
Khan lent his help to Cursed Station in 2033 because he didn't want it to end up like Polyanka.In the first game, it's not really explained why Khan is so invested in helping defend Cursed Station from the Nosalis hordes— yes, there's plain old altruism, but the revelations about Khan's past from the Chronicles Pack in Last Light cast the entire incident in a new light. Notice the simililarities in game play between "Cursed" in 2033 and the Khan DLC mission in Last Light— in both, you are tasked with blowing up the entrances to a station that has been totally overrun by mutants in a last-ditch attempt to keep more from coming through. Now try and imagine that similarity from Khan's point of view— he remembers Polyanka all too clearly, and he's watching the exact same thing unfold at Turgenevskaya/Cursed. The difference this time, though, is that he has both experience and Artyom's help on his side— thus, at least some of the people from Cursed were able to be saved. This also explains why Khan says that the whole thing with the ghosts was "very personal" to him— the ghostly squad in the tunnel, while not his original buddies from Polyanka (which would make no sense, given the geographical distance), are still friends of his who died trying to save Cursed from the fate that befell his own home. To Khan, saving Cursed wasn't just the right thing to do— it was a chance to right a personal wrong and make peace with the past.
Metro 2033 takes place in the same Russia as Stalker.The Stalkers and anomalies speak for themselves. After Call of Pripyat something in the Zone made everyone freak out and start a nuclear war, or the rest of the world wanted the Zone destroyed. Instead the Zone expanded over all of Russia.
Khan is a JediOr at the very least he's Force-sensitive. Nobody really knows who or what Khan is or where he comes from, and is described as a wizard or magician in both the games and the original books. In the book, he even pulls off what amounts to a Jedi mind trick, and he has a much greater understand of the spiritual than probably any other inhabitant of the Metro. He may not have a lightsaber, but that doesn't make him any less of a Jedi.
The "Little Dark One" from Last Light is a double-mutantCompare the Little Dark One to any other member of his species, and you'll see how different he is: first and foremost, he actually has a mouth, unlike the other Dark Ones you can see and encounter. This is probably because, in addition to being mutated from humans like the rest of the dark ones, he underwent secondary mutations different from the rest of the Dark Ones.
The Spiderbugs are descended from UngoliantAs if the Spiderbugs weren't scary enough already! If you go on the idea that they're not actually mutants but ancient monsters emerging now since humanity doesn't dominate the Earth anymore, they're probably the distantly-descended spawn of Ungoliant, one of the few beings in Tolkien's legendarium that has no explanation whatsoever and is so evil that she tried to eat MORGOTH.
Artyom's drink at the bar in Theatre was spikedWhile it appears to be that Artyom Can't Hold His Liquor, in the first game he downed several shots of vodka with no adverse effects. While you could make an argument with the Bullet at the bar in Venice, that one is probably just really strong. Since Pavel is deliberately trying to incapacitate Artyom in this scene, he must have taken extra measures to make sure Artyom was out like a light.
Someone found a huge box of cheap laser pointers on a train somewhere, and this is responsible for all the laser technology in the metroThe gun salesmen are ripping you off when they claim it's a "pre-war laser sight" and charge 15 MGRs to mount it to your weapon. Someone also hacked these into the laser-based trigger mechanism of the much-more-reasonably priced claymore mines.
This game is in a shared universe with Adventure Time, albeit separated by a large span of timeAdventure Time has, in its distant past, a "Great Mushroom War" that "brought the magic back into the world". Pretty much the same thing happened here, though it's only 20 years afterward.