So this has bugged me for a long time. I'm a physicist in training, and have very little knowledge of the cosmos, but: I've heard for a long time the notion that the universe is infinite. For a long time, this troubled me, because I really couldn't conceptualize how something could be infinite, and at the same time, how we would truly even be able to measure that something is infinite. Then I hear that the universe is expanding.... Something that is infinite, cannot expand. Finally, I constantly hear big numbers, of calculations of how many galaxies there are in our universe, and how many protons and how many stars, etc etc. If our universe was infinite, there would be an infinite number of stars/galaxies/protons. I'm not high enough in mathematics just yet to really grasp it, but my first thought of this is that it's "mathematically" infinite. That is, for all our calculational purposes, we just assume it is. That's just a wild guess though If someone could explain this, that would be fantastic.