Zekkyou on 10 March 2015
It's not really a 'game' in the traditional sense, but Space Engine is essentially an open universe and has been in development for about 10 years :p Here's a pic to give you an idea of how big it is:
The first pic shows a laughably tiny number of the galaxies in SE, and every one of them is filled with millions of stars, planets and moons that you can travel to. Makes you feel pretty small '-'
A video showing some of the cool places people have found: https://www.youtube.com/watch?v=b1O3XCsGnC8
