Dark Energy III 
When you brew up a universe, a tad too much of a key ingredient -- or a smidge too little -- can change the whole thing.
Consider the "ingredient" known as dark energy. It's a mysterious force that's causing the universe to expand faster as it ages, and it makes up almost three-quarters of all the matter and energy in the universe.
Dark energy was discovered a dozen years ago, but scientists are still studying its true nature -- an energy from space itself, a flaw in our understanding of gravity, or something else. Whatever it is, one thing is certain: the wrong amount of it would have profoundly changed the universe.
In fact, if there was too much dark energy, it might have prevented everything we see in the universe from forming, as University of Texas physicist Steven Weinberg explains:
WEINBERG: It is the largest contributor to the energy of the universe. If it were much larger, it would have interfered with the formation of the galaxies, because the accelerated expansion of the universe would have torn the galaxies apart before they could form. It is gradually becoming more and more the dominant source of the energy in the universe.... We're just now at a point of history in the universe where the dark energy has just begun to take over. And if we wait another hundred billion years, the effect of matter on the expansion of the universe will have become negligible.
We'll talk about a search for dark energy tomorrow.
Script by Damond Benningfield, Copyright 2010
Today's program was made possible in part by the Texas Cosmology Center.