Third thing: probing the very small. Now, to explain this, I need to tell you a very disturbing fact. Most of the universe's matter is not made up of atoms. You were lied to. Most of it is made up of something very, very mysterious, which we call dark matter. Dark matter is something that doesn't like to interact very much, except through gravity, and of course we would like to learn more about it. If you're a particle physicist, you want to know what happens when we smash things together. And dark matter is no exception. Well, how do we do this? To answer that question, I'm going to have to ask another one, which is, what happens when galaxy clusters collide? Here is an image. Since galaxy clusters are representative slices of the universe, scaled-down versions. They are mostly made up of dark matter, and that's what you see in this bluish purple. The red represents the hot gas, and, of course, you can see many galaxies. What's happened is a particle accelerator at a huge, huge scale. And this is very important, because what it means is that very, very small effects that might be difficult to detect in the lab, might be compounded and compounded into something that we could possibly observe in nature. So, it's very funny. The reason why galaxy clusters can teach us about dark matter, the reason why galaxy clusters can teach us about the physics of the very small, is precisely because they are so very big.