It’s called the Fermi paradox, named after physicist Enrico Fermi. It refers to the apparent contradiction between the lack of evidence and high probability for the existence of extraterrestrial civilizations. Finally, the Journal of the British Interplanetary Society has an explanation, which it has released in a paper entitled The Aestivation hypothesis:
Maybe we are not seeing alien civilizations because they are all rationally “sleeping” in the current early cosmological era, waiting for a remote future when it is more favourable to exploit the resources of the universe. We show that given current observations we can rule out a big chunk of possibilities like this, but not all.
Makes sense to me. But then, spec fiction fans have guessed this to be the case for quite some time:
“Ph’nglui mglw’nafh Cthulhu R’lyeh wgah’nagl fhtagn.”
Yep. That explains it.