[pfdietz]

It's rare in biological systems because it's terminated by running out of some resource.

But you're saying galactic colonization would terminate without running out of new systems to colonize.

There would be a slowdown due to geometric constraints -- only so many new systems adjacent the boundary of the colonized zone -- but that hardly solves your problem.

[runarberg]

My speculation is that the size of space is an obvious geometric constraint which will limit the span of any civilizations almost immediately.

If we look at humans, we have both the space, technology, and the resources to expand even further on earth, yet our span only marginally larger then it was 10 000 years ago. We can have permanent settlements on Antarctica, floating on the ocean, etc. but we don’t. We can increase our population by another order of magnitude, but again, it looks like we won’t. This follows the same population dynamics as most other species on earth. I think aliens will be no different.