The official description on the main page reads: "SpaceEngine - 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."
The first thing I'll say about it is that you will need a decent graphics card to run it, I have an ATI Radeon HD 5570 with 1GB of VRAM, so not amazing, but decent enough, and I can run it on the highest graphics settings, with... Quite a bit of lag but due to the nature of the simulation that's expected honestly. (Oh and also, there are no loading screens other than the initial game startup! It's awesome.)
So anyways, more about the actual program itself. (Notice I hesitate to call it a game, it's more of a simulator.) Space Engine will use actual astronomical data and probabilities to procedurally generate an entire universe for you to explore, at a total size of 10 x 10 x 10 gigaparsec's (32,616,334,400 lightyears cubed, keep in mind the Milky Way is approximately 100,000 - 120,000 lightyears in diameter.) This universe contains millions if not billions of galaxies, each one containing hundreds of billions of stars and each one of those with up to and upwards of 20 planets, and even further each one of those planets with up to and upwards of 10 moons. The kicker is that on maximum settings... It can look like this. (Disclaimer: I do not own the video, all credit to the maker.)
Or see the SpaceEngine screenshot gallery on the official website.
Deep Space: http://en.spaceengine.org/photo/deepspace097/9
So now with all of that being said, I will close with the following quote taken from the man himself who created the program, SpaceEngineer.
(And yes, this program was made by ONE person!)
EDIT: Another thing I would like to mention is that if your in the program and try to land on earth or any other bodies in the "Sol" system, the terrain will not be as visually pleasing as the procedurally generated planets, this is because it is using actual data to try and replicate the real thing, which because of the size of a planet, is fairly hard to do in excessive detail. The reason procedurally generated planets are so detailed is because the computer has free reign to create whatever it wants in as much detail as it can possibly create.Anyway, I should note: as long as SE is a scientifically accurate Universe simulator, then one day it will implement ALL space objects and phenomenon that are known to modern astronomy. So you may not bother yourself with questions like "will SE one day have meteor rain implemented". I guarantee that it WILL.