Dark Energy 
Take a look around you. Notice your hands, the trees outside your window, the Sun in a blue sky. It's the universe we know -- the things we see and feel, the things that make up stars and planets and our own bodies. Yet it's little more than a rounding error in the cosmic ledger. That's because normal matter and energy account for only a tiny fraction of the universe. Steven Weinberg, a Nobel Prize-winning physicist at the University of Texas, explains:
WEINBERG: The energy budget of the universe as it is now is about four percent ordinary matter -- that is, ordinary atoms, you, me, the Sun, the Moon, the stars; about 23 percent dark matter, which is matter but not ordinary matter, not ordinary atoms, but some kind of particles; and all the rest, about 70-some percent, is dark energy.
Astronomers discovered dark energy in the 1990s. They were measuring how the expansion of the universe changes. They expected to find that, over time, the rate of expansion slows down -- the result of the gravitational pull of matter.
Instead, they found that the universe is expanding faster than it did in the past -- as though someone were pushing down on a cosmic accelerator. Scientists couldn't explain what was causing this acceleration, so they called it "dark energy."
Continued observations have shown that it makes up most of universe. Yet no one can say for sure what it is -- although there's no lack of ideas. More about that tomorrow.
Script by Damond Benningfield, Copyright 2010
Today's program was made possible in part by a grant from the NASA Science Mission Directorate.