Summary: speaker show some images of clusters of galaxies. They are huge, beautiful collection of galaxies, but more than that galaxy clusters are mysterious, surprising and useful, useful as the most massive laboratories. There are four major types. The first type is very big. He showed an image of a particular galaxy cluster which has a mass of over one million billion suns, and they are essentially isolated systems so we can think of them as a scaled-down version of universe. We might know how gravity works by studying these systems. The second one is very hot. It’s hot gas, plasma. The great speeds made the great temperatures. But we still need understand more about the physics of the hot. The third thing is probing the very small. Most of the universe's matter is not made up of atoms. Most of it is made up of something very, very mysterious, which we call dark matter. 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. The fourth thing is the physics of the very strange. Cosmologists understand why the universe is expanding. They don't understand why it's expanding at an ever-increasing rate. The reason why he talks about clusters of galaxies is that we need to learn more about it. The problem with the large-scale structure of the universe is that it's horribly complicated. We have many difficult problems, and the measures are not obvious. We need to focus, to concentrate these things come when we broaden our field of vision when we step back when we zoom out. So he can't think of a better way to do this than by studying the universe.