[Ed Kolis]

What aliens? Heck, what humans? The population of the universe is zero! See, space is infinitely big, right? So it has an infinite number of planets. But not all planets have life, so only a finite number of planets have life. A finite number divided by infinity is zero, so therefore zero percent of the planets in the universe have life. Therefore the population of the universe is zero.

Seriously though, I think that any aliens that visit Earth would probably be friendly - any race developed enough to have interstellar travel would probably not be warlike, or they would have destroyed themselves sooner. On the other hand, we're on the verge of achieving spaceflight, and we're pretty warlike...