And I think there are four major types, and the first type that I want to describe is probing the very big. So, how big? Well, here is an image of a particular galaxy cluster. It is so massive that the light passing through it is being bent, it's being distorted by the extreme gravity of this cluster. And, in fact, if you look very carefully you'll be able to see rings around this cluster. Now, to give you a number, this particular galaxy cluster has a mass of over one million billion suns. It's just mind-boggling how massive these systems can get. But more than their mass, they have this additional feature. They are essentially isolated systems, so if we like, we can think of them as a scaled-down version of the entire universe. And many of the questions that we might have about the universe at large scales, such as, how does gravity work?might be answered by studying these systems.