SpaceEngine is, without hyperbole, one of the most breathtaking pieces of software ever created. It is a realistic, interactive 3D planetarium and universe simulator that allows you to explore the cosmos from the surface of Mars to the accretion disk of a black hole billions of light-years away. Unlike "games" like No Man's Sky, SpaceEngine uses real astronomical data and procedural generation to create a universe of trillions upon trillions of stars, galaxies, nebulae, and planets.
You can obtain the free "legacy" versions through the following sources:
Even the free version is graphically intensive because it renders the entire universe in real-time. To run it smoothly, you’ll generally need: Windows 7 or higher.
1. Free version (older, unsupported)