IMO the 2 best open space games aren't finished yet!
Elite Dangerous, and Star Citizen are what you need to keep an eye on.
In the meantime, heck I'd either play an Evochron series game, or start making your own space game. Seriously, making your own space exploration game is about as much fun as playing one - at least for a developer IMO. If it's the exploration that you crave, then what better place to explore than your own world!
That's how I think about it at least - I want a really epic space game as well, but there's nothing that comes close until those aforementioned games are finished, so why not take matters into your own hands.

I am the one who knocks...
