probability

If you're eating toast, and you accidentally bump it to the floor, it seems more likely to land buttered side down. This is one of the most common formulations of Murphy's Law--the tongue-in-cheek axiom that states "If something can go wrong, it probably will."

According to reasonable estimates of how many stars have planets, how many planets may be suitable for the evolution of organisms, and so on, our Milky Way Galaxy should be brimming over with life.