The Space Engine is

a freeware space simulation software that lets you explore the universe in three dimensions, starting from planet Earth to the most distant galaxies. Areas of the known universe are represented using actual astronomical data, while regions uncharted by astronomy are generated procedurally. Millions of galaxies, trillions of stars, countless planets – all available for exploration.

Here are a couple of videos describing the program:

Find more space sim links in the HobbySpace Space Simulations section.