Today, I learned about Fermi's Paradox. For the non-pop sciences in the crowd, the paradox says that if Earth is reasonably normal in the wider scope of the galaxy, why haven't we come into contact with any space-faring aliens yet. Occam's Razor suggests two immediate possibilities: first, that the reason we haven't found any extraterrestrial life is that there isn't any. And second, that they, like us, have the basic technology to spread out, but really don't see the point in venturing much beyond our immediate vicinity.
I prefer the Sea Monkeys theory: we are the pets of a much greater race of beings and they'd really like to open up the tank and get a closer look, but they don't want to void the warranty.