The Great Filter is the idea that there is some single, almost insurmountably improbable barrier on the path to the stars that explains why we've never seen any sign of alien life.  It combines aspects of astrology, biology and history to arrive at one inescapable conclusion: university professors dream of book deals.

Robin Hanson of George Mason University posits a "Great Filter" that prevents the rise of intelligent, self-aware, technologically advanced, space-colonizing civilizations. The "filter" would be one or more improbable steps along the path that starts with the creation of a planet and ends with a race capable of colonizing the galaxy.

Somewhere between those two points, philosopher Nick Bostrom points out, "the Great Filter operates, and it must be powerful enough that even with all the billions of possible starting worlds on which life might evolve - all those rolls of the cosmic dice - one ends up with nothing: no aliens, no spacecraft, no signals,