Where are All Those Aliens?

COSMOS - Most scientists assume that the universe must be populated with innumerable alien intelligences and civilizations—after all, we humans can't be so special. OK, so where are they, these "innumerable alien intelligences and civilizations"? How come there's zero evidence?

Featuring Jill Tarter, Douglas Vakoch, Frank Drake, Raymond Kurzweil, Francisco Ayala, Steven Dick, and David Brin.