Space

We are indeed limited in what portion of the observable universe is reachable thanks to the expansion of the universe. IIRC something like 95% of the galaxies we can see would be impossible to ever reach even if we travelled at the speed of light to get there, they'd disappear across the cosmological event horizon before we could get there.

But 5% of the observable universe is still pretty darned big. I wouldn't be surprised if our expanding sphere of colonization eventually bumped into another one. But we'll have colonized hundreds of millions of galaxies by then, so I'm not going to be too upset by that. Mustn't get greedy. :)

You might be interested in this article, Eternity in six hours. It suggests that a civilization with access to six hours' worth of a single Dyson swarm's total energy output could launch a colony ship to every galaxy within that reachable horizon. This makes the Fermi paradox a lot sharper since aliens could do the same thing, but it really illustrates just how easy it should be to get spreading around the cosmos once we get up into space in a serious way.