Cosmology is the study of the overall structure of the universe. Just what is the Universe? It is everything that exists. However, from Earth we cannot observe everything in the Universe. Some things are dark (brown dwarf stars, planets, and Dark Matter) and we cannot see them. Additionally, there are parts of the universe whose light has not yet reached us in this part of the Universe. And because light travels at a set speed we actually look back in time when we look into the cosmos.
Astronomers observe some interesting structure in the Current Universe. That structure can tell us much about the History of the Universe. It can also tell us what we can expect for the Future of the Universe and beyond...

A computer simulation depicting a large chunk of our universe