For those interested enough to pick up a good book on the subject:
Where is Everybody?: Fifty Solutions to the Fermi Paradox and the
Problem of Extraterrestrial Life
by Stephen Webb
Publisher: Copernicus Books
ISBN: 0387955011
Publishing Date: 2002
Pages: 288
Format: Hard cover
Given the fact that there are perhaps 400 million stars in our Galaxy
alone, and perhaps 400 million galaxies in the Universe, it stands to
reason that somewhere out there, in the 14-billion-year-old cosmos,
there is or once was a civilization at least as advanced as our own. The
sheer enormities of the numbers almost demand that we accept the truth
of this hypothesis. Why, then, have we encountered no evidence, no
messages, no artifacts of these extraterrestrials? Webb discusses in
detail the 50 most cogent and intriguing solutions to Fermi's famous
paradox: If the numbers strongly point to the existence of
extraterrestrial civilizations, why have we found no evidence of them?