Dark Energy II 
The universe is full of mysteries. The biggest one is what makes up almost three-quarters of the universe: dark energy. It causes the universe to expand faster as it ages. Yet it was only discovered a little more than a decade ago. And so far, there's no agreement about what it is.
HILL: Well, the problem with dark energy is that really there are too many possible theories for it. Right now, we're in a situation where the theorists sort of know no bounds. We've got to provide them with some boundary conditions.
That's Gary Hill, an astronomer at McDonald Observatory and one of the leaders of a team that's searching for dark energy. Another team leader is Karl Gebhardt:
GEBHARDT: Dark energy could be just a property of space itself, and that's the number one issue which people are considering at this point. And this is called the vacuum energy. Just the existence of space -- because space is there -- there may be energy associated to that space. And the bigger space is, the more energy there is.
And team member Niv Drory offers another possible explanation:
DRORY: Another possibility is, which is quite interesting, however unlikely, is that there is something subtly wrong with Einstein's Theory of General Relativity, which governs the behavior of the universe on large scales because it describes gravity and gravity is the only force relevant in the universe on large scales....The other possibility is that General Relativity is correct, we just are not applying it correctly to the universe.
More about dark energy tomorrow.
Script by Damond Benningfield, Copyright 2010
Today's program was made possible in part by the Texas Cosmology Center.