I find it strange that this “game” hasn’t been mentioned in these forums around.
Ever heard of “SpaceEngine”?
It is a procedural generated simulation of the universe (for real though), based on physics, astronomical observation as well as NASA data about real space objects. So, what does really exist is being simulated and the rest is being procedurally generated.
The program has been in development for several years (I think development began in 2005), with its first release in 2010. It should be mentioned that is being developed by one guy only.
To put it simply, it is like No Man’s Sky on god mode, without flora or fauna and no gameplay mechanics, but with proper physics. You can roam freely in the entire universe (visit every planet, moon or asteroid in every system of all 13 quintillion galaxies that there are) and adjust time to your content. It also features ships you can spawn and fly around with them, but the flight mechanics are very complex.
Here’s a preview of the current version available, 0.9.8.0:
And here’s a preview of the version that will follow, 0.9.9.0, which also will be available on Steam:
So if you like exploring the wonders of the universe, you should definitely give it a try, since it is freeware.
Like No Man’s Sky, I’ve put hundreds of hours into that program.