Starsector (formerly Starfarer) is still in development but it's been a seriously impressive game for years. Not yet on Steam or any other store, as you have to buy it direct, but don't let that stop you with a big update out now.
Oh my, you're right! I thought it's in unity, because of their later strong stance against it. Maybe PC version is in Java and some of the ports in Unity?
Why would that be the first reason you came up with and not the fact that steam takes a ridiculous 30% cut from dev's work? And yes, the game is that good, so good they can tell steam to fuck off. Makes me like the game even more..