What if, just what if interstellar travel really is nigh onto impossible and what if stuff like AI, Von Neuman replicators and 'singularities' are simply not possible? Thus, we'd have no intelligences spanning solar systems, never mind pervading the galaxy. We're mostly SF buffs and folks bullish on technology but a rational person must concede that these common techie postulates are not guaranteed truths. Because of our cultural predilections we space cadets tend to accept these future technical developments unquestioningly, almost as articles of faith.

Another point. What if protiens, self-replication, the start of life really was a once in the universe or galaxy improbability as many notable biologists (not astronomers) have argued? Or what if the transition to multi-cellular life (it took billions of years here) is also another highly improbable occurance even on a galaxy wide scale? We have no real objective evidence as yet to guide us here.