From Edge:
[SEAN CARROLL:] Why does the universe looks the way it does?
This seems on the one hand a very obvious question. On the other hand, it is an interestingly strange question, because we have no basis for comparison. The universe is not something that belongs to a set of many universes. We haven't seen different kinds of universes so we can say, oh, this is an unusual universe, or this is a very typical universe. Nevertheless, we do have ideas about what we think the universe should look like if it were “natural”, as we say in physics. Over and over again it doesn't look natural. We think this is a clue to something going on that we don't understand.One very classic example that people care a lot about these days is the acceleration of the universe and dark energy. In 1998 astronomers looked out at supernovae that were very distant objects in the universe and they were trying to figure out how much stuff there was in the universe, because if you have more and more stuff — if you have more matter and energy — the universe would be expanding, but ever more slowly as the stuff pulled together. What they found by looking at these distant bright objects of type 1A supernovae was that, not only is the universe expanding, but it's accelerating. It's moving apart faster and faster. Our best explanation for this is something called dark energy, the idea that in every cubic centimeter of space, every little region of space, if you empty it out so there are no atoms, no dark matter, no radiation, no visible matter, there is still energy there. There is energy inherent in empty spaces. We can measure how much energy you need in empty space to fit this data, this fact that the universe is accelerating. This vacuum energy pushes on the universe. It provides an impulse. It keeps the universe accelerating. We get an answer and the answer is 10-8 ergs per cubic centimeter, if that is very meaningful.
More here. (Note: You can also enjoy watching this intelligent conversation with the brilliant and masterful Dr. Carroll on video)