Space opera wise I found something that may be interesting.

Doing a brief calculation I say there is every 100000 years about 25 civilizations with spaceships, plasma guns, energy shields and warp drive based FTL, and each of these civilizations on average takes 1000 star systems.

Only around 1,250,000,000 star systems would be colonized assuming a 5 billion year span for intelligent space faring societies and total uniformity to base64 math. That's not much given the galaxy has 300 billion stars.

So maybe it really may just be that the universe, let alone the milky way galaxy is just really, really big.

Although there lies a new issue; What about self replicating probes? A civilization that used such probes would be able to easily comb the galaxy in a mere 10 million years.