Wormhole

Space turned out to be much more crowded than we expected, thanks to wormhole generation, which like most species discovered by accident as we were working on other things.

We learned quickly how we were expected to act, but unfortunately there were, as always, a bunch of assholes who wanted to bully, steal, and generally act horribly.

The other species were understanding, at first, but after we blew up a star, they declared that we were unfit to exist in the galaxy. We feared they would deny us wormhole tech.

We didn't know wormholes could be planet sized, nor inter-universe...