Fourth thing: the physics of the very strange. Certainly what I've said so far is crazy. Okay, if there's anything stranger I think it has to be dark energy. If I throw a ball into the air, I expect it to go up. What I don't expect is that it go up at an ever-increasing rate. Similarly, cosmologists understand why the universe is expanding. They don't understand why it's expanding at an ever-increasing rate. They give the cause of this accelerated expansion a name, and they call it dark energy. And, again, we want to learn more about it. So, one particular question that we have is, how does dark energy affect the universe at the largest scales? Depending on how strong it is, maybe structure forms faster or slower. Well, the problem with the large-scale structure of the universe is that it's horribly complicated. Here is a computer simulation. And we need a way to simplify it. Well, I like to think about this using an analogy. If I want to understand the sinking of the Titanic, the most important thing to do is not to model the little positions of every single little piece of the boat that broke off. The most important thing to do is to track the two biggest parts. Similarly, I can learn a lot about the universe at the largest scales by tracking its biggest pieces and those biggest pieces are clusters of galaxies.