[ohyeshedid]

> If they can build Dyson spheres, wouldn't they already be all over the place?

I figure they'd have to be.

They'd need more than a solar system's worth of raw resources to build the sphere. They may need more than a solar system's worth of resources just to build the tools and craft to be able to build the sphere. They're going to need an insane amount of production facilities for various components, and that's going to take a massive amount of resources. That's going to require exploring quite a bit of a galaxy.