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Pixel Art refers to the charming, simplistic, retro/renaissance style of very-low-resolution graphics, such as from the 1977-1995 Atari, GameBoy, & Super Nintendo (SNES) eras.

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

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

Games where the primary goal is keeping the player character alive under harsh circumstances such as dwindling resources or a hostile environment.