People typically say "the galaxy is big, so it's unlikely", which is not necessarily true. You need no more than a few tens of thousands of years to colonize a

People typically say "the galaxy is big, so it's unlikely", which is not necessarily true. You need no more than a few tens of thousands of years to colonize a significant part of the galaxy, and it's not unthinkable many societies have had that kind of head start. The real expanse that separates civilizations is time. It takes only slightly more than 1000 years to travel a thousand light years, but the gaps in time can be much bigger. It's possible that over the course of a billion years, many thousands of civilizations rise and fall, but at any given time only a handful are in existance concurrently.