Space Engine – the universe simulator

[Windows Only] SpaceEngine is realistic virtual Universe in your computer. You can travel from star to star and from galaxy to galaxy, landing on any planets, moons and asteroids and explore alien landscapes, you can change the speed of time flow and observe celestial phenomena. All transitions are completely seamless, the universe has a size of billions of light-years across and contains billions of planetary systems. Procedural generation is based on real scientific knowledge, so SpaceEngine depicts the universe the way it is thought by modern science...

http://new.spaceengine.org