itch.io

Uses two-dimensional "sprites", 2D images created and used on a flat plane, as opposed to the three-dimensional models or environments found in 3D games.

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

Games which have short playtimes and can usually be completed in one sitting.

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