The
Drake equation (well worth a read). It almost seems an obvious conclusion once you grasp just how enormous the universe is. Per the article, with the original values Drake used, he estimated there were 10 detectable extraterrestrial civilizations in the entire Milkey Way galaxy. Now realize there are billions of other galaxies in the universe. So yeah, how could there not be other life in the universe?
But as Rufus said, time and distance is a bitch. There are some interesting theories related to the Drake equation (e.g.
Fermi paradox) that try to explain why, if extraterrestrial life should be so common, we haven't detected any yet. Seems like the crux of it is, technological civilizations don't last very long. Even a 10,000 year old civilization is a drop in the bucket of the estimated 14 billion years the universe has been around. So yeah, between the relatively short lifespan of an advanced civilization and the huge distances between galaxies (hundreds of thousands of light years), the chances of bumping into each other does seem pretty small. =/