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Games in which the player shoots things to progress, usually with a variety of guns, and other times with bows and arrows.

Typically shoot em' up games where the emphasis is dodging a large number of bullets on the screen at once. Bullets form dense patterns on the screen that require precise controls to avoid.

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.