Geoffrey Miller has a theory about Fermi's paradox. Seventy years or so ago, a bunch of physicists were wondering about the plausibility of extra-terrestial intelligence. It was obvious already that there were lots of stars, and it seemed likely that many of them had planets. Once intelligent life had evolved, it shouldn't take long to colonize a galaxy.
Fermi listened patiently, then asked, simply, “So, where is everybody?” That is, if extraterrestrial intelligence is common, why haven’t we met any bright aliens yet? This conundrum became known as Fermi’s Paradox.
The paradox has gotten somewhat sharper since. We have now discovered hundred of extraterrestial planets, and it's clear that they are pretty common. It's plausible, if not yet demonstrated, that there are many which are good candidates for supporting life. The evolution of life and especially intelligence is more problematic - it took a long time here on Earth. Still it's plausible - again not yet demonstrated - that there are tens of millions of planets in our galaxy that have been hospitable for life for much longer than the history of the Earth.
So where are they? For Miller's theory, visit the link.