Too many worlds. That’s the problem with space. You develop interstellar flight and hope to find a big emptiness that you can coast around in until all of the stars fade to black, but there’s all this stuff scattered about. Planets and asteroid belts and big alien jellysquids.
Stellaris‘s upcoming expansion, Apocalypse, will let you clear out some of the clutter. It brings planet-destroying weapons into the game, along with new Titan capital ships, massive orbital installations and marauding space nomads, who can be recruited to your cause, but can also trigger a new late-game crisis. There’s some non-violent stuff as well for the gentler souls among you.