The Fermi Paradox

The Fermi Paradox can be defined thusly:

The size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist. However, this belief seems logically inconsistent with the lack of observational evidence to support it. Either the initial assumption is incorrect and technologically advanced intelligent life is much rarer than believed, current observations are incomplete and human beings have not detected other civilizations yet, or search methodologies are flawed and incorrect indicators are being sought.
(wikipedia).

The Space Review talks about a book on the paradox by Stephen Webb, where he delineates 50 possible solutions to the paradox, from autonomous alien weapons to the fact that we're alone.

There is also a new episode of the Universe Today podcast on the Fermi Paradox.