[Beasty]

Well, it isn't infinite per se, but supposedly it is infinitely expanding. There are two main theories; first that our universe is composed of a finite amount of matter expanding away from the origin point into an endless perfect vacuum such that the "edge" of the universe is the farthest point from the origin point where matter exists at any given time.

The other idea is actually kind of strange. It posits that from our perspective the universe is a 3 dimensional plane, but that in actuality there are more dimensions, resulting in our universe being more like a still image drawn on the surface of a balloon that is getting filled with more and more air. From the point of view of a dot drawn on the balloon (us), a second dot drawn a cm away would be moving away at an increasing rate, but in actuality the total surface area of the universe is merely expanding while the data points on the balloon stays in the same position relative to the original size. The way to test this would be to approach the "edge of the universe" at any given time and see if light emitted from stars on the opposite end of the universe are visible to us in that direction, which is currently a physically impossible task as you would need FTL technology to get close enough.

The second theory isn't as crazy as it sounds, string theory predicts more than just 3 dimensions.