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Games that try to simulate real-world activities (like driving vehicles or living the life of someone else) with as much realism as possible. Simulators generally require more study and orientation than arcade games, and the best simulators are also educational.

Games that can be played by more than one player, either locally or online.

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.