Of course the underlying assumption(s) to all of this speculation is that any 'intelligent' race would want to expand across the cosmos. And further that civilizations would remain stable enough to do so. Why do we assume that? Maybe most races are content to remain on the planet where they evolved -- or at least in the same star system -- and solve all of the riddles of mathematics and philosophy. Or play video games...
Or maybe they are all like us and too busy competing with and killing each other to cooperate long enough to even get out of their own atmosphere...
