Here's a scary fact: Once humanity figures out how to reach and colonize our nearest star system, assuming it takes us 1,000 years each time we colonize the next closest star systems, it would only take 25 million years to colonize the entire galaxy. 25 million years is nothing. So why isn't the entire galaxy teeming with intelligent life? Our universe has been around for 13 billion years - plenty of time for this to have taken place.

The most likely answer (and one we don't want to admit to ourselves) is that intelligent species have a certain lifetime limit. Most species die off within 3 million years.

An alternative answer would be that advanced intelligence figures out a way to delve into mind or other dimensions, rather than outwards into physical space.