Almost everyone in this thread believes that because there are billions of stars in each galaxy, and billions of galaxies in the observable universe, even a small probability of life arising would mean a great abundance of life throughout the universe. The classic argument that the Drake equation can be satisfied even with an exceedingly small f1 and fi. However, we can flip this argument on its head by asking: where are the aliens? 10 billion trillion stars, over 13 billion years, and not one of them has developed a species that has sent radio waves that can reach Earth. The radio silence of the universe is telling. The fact is, it is nearly impossible that zero discernable signals would have been emitted within that time, and at that scale, unless the probability of developing (intelligent) life is astronomically low. The fact that there is an incredible number of habitable worlds, yet no observed aliens, is the greatest proof that there is nothing out there.
I'd like to state that there is nothing I would like more than to be proven wrong. These facts sadden me immensely.