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Near infinitely replayable games that usually feature, random generation, perma-death, and brutal difficulty. 

Science-fiction. Commonly including things like aliens, advanced technology, and settings of outer space or distant planets. 

Shoot 'Em Up or shmup games are a subset of shooter games where the player continually moves forward while waves of enemies enter the screen. Both shooting and dodging are critical to advancing. See also Bullet Hell.

The larger universe outside of Earth's atmosphere. Sometimes really, really far outside of it.