The Drake equation is a theory, developed by astronomer Frank Drake in 1961, purportedly estimating how many extraterrestrial civilizations are likely to exist in the universe.
If you think about the gazillions of stars in the universe; and if you think about the gazillions and gazillions of planets likely circling these stars (the last five years, thousands have been discovered in our solar neighbourhood only!), the chance of alien life not existing must be very tiny. And the odds of intelligent life and civilizations developing must therefore also be high, just looking at the statistics of the universe.
The BBC has a very cool interactive infographic exploring the Drake equation. It also includes its detractors: after all, if alien civilizations must exist, why hasn’t any sign of them been found yet? The universe is already 13.5 billion years old, after all. Chances are that no civilization has ever developed to the point of interstellar communication without first destroying itself… More here.