What are the dark forces in the game Metro 2033?

The novel or the game doesn't give an answer, so readers can only guess.

1 alien immigrants? Like Columbus's discovery of the new continent, it seems that aliens really like thermonuclear reactions, and the United States, a big thermonuclear reaction country, often encounters such events.

Non-human variant? A new race, with advanced wisdom, pure spiritual communication, enjoying memory and knowledge, is really productive, similar to bee colony and ant colony.

3 human variation? In order to survive on the surface full of monsters, human beings gave up language and used eyes, expressions, gestures and collective actions, and finally evolved spiritual communication.

4 Pre-war experimental body? At the end of the game Dawn, there are still dormant black monsters in the D6 base, which makes people wonder whether this fighting clan is still experimenting with biochemical arms and how a large number of carnivorous monsters on the surface can survive without herbivores.

Five creatures of different dimensions? Khan said: human nuclear bombs destroyed heaven and hell. It makes space chaotic, the balance between material space and spiritual space is destroyed, a large number of ghosts wander in the real world, and there are terrible lightning balls. Maybe the black monster is just the guardian of balance.