Dark energy is the name given to an unexplained force that is drawing galaxies away from each other, against the pull of gravity, at an accelerated pace.

Dark energy is a bit like anti-gravity. Where gravity pulls things together at the more local level, dark energy tugs them apart on the grander scale.

Its existence isn't proven, but dark energy is many scientists' best guess to explain the confusing observation that the universe's expansion is speeding up. Experts still don't know what's driving this force, but the quest to learn more about dark energy is one of cosmologists' top priorities.

Confounding expectations

The story of how dark energy was discovered is a classic case of science confounding expectations.

In the mid-1990s, astronomers set out to measure how fast the universe was expanding. Because gravity draws mass together, most experts expected to find that gravity had slowed down the universe's rate of ballooning, or perhaps that the rate was staying about the same.

Instead, it appeared that the expansion was doing neither: It was speeding up.

"The data wasn?t behaving as we thought it would. There was a lot of nervous laughter," said Brian Schmidt of the Australian National University in Canberra, who led a team along with Johns Hopkins University astrophysicist Adam Riess that helped discover dark energy in 1998.

The evidence was based on measurements of bright exploding stars, called supernovae, that astronomers were using as lampposts to track distance. By looking farther away, scientists are able to peer back in time, since the light from distant objects has taken billions of years to reach us.

The scientists observed many supernovae at different distances to determine how fast they are speeding away from us. (They measured the objects' red-shift, or how much their light had been changed due to the Doppler effect, which is the compression or expansion of waves that occurs when an object is moving toward or away from you. An analogy is the siren of an ambulance that changes pitch as it moves toward you, then passes you and heads the other way ? its waves are first compressed, then stretched.) These measurements gave astronomers a picture of how fast the universe was expanding at different points in its history.

Shocking results

The researchers also found that the universe is expanding faster today than at any time in the past.

"At first we were reluctant to believe our result," said Saul Perlmutter, an astrophysicist at the Lawrence Berkeley Laboratory of the University of California, Berkeley, who led a competing team that found the same results as Schmidt and Riess. "But the more we analyzed it, the more it wouldn't go away."

To explain these puzzling findings, some scientists have revived an old idea of Einstein's that had been discarded as false: that the vacuum of space has energy in it that acts repulsively and accelerates the expansion of the universe. Einstein called this idea the cosmological constant, and referred to it as his "biggest blunder."

Now the cosmological constant is one of the leading theories of why the universe is blowing up like a balloon at ever-increasing speed.

Dark matter

Dark energy is sometimes confused with the similarly mysterious dark matter, though the two are separate entities.

Dark matter is a hypothesized form of matter that doesn't interact with light, so it is invisible. Astronomers deduced its presence by noting its gravitational pull on stars in galaxies.

Taken together, dark matter and dark energy seem to make up most of the mass of the universe (matter and energy are considered to be two forms of the same thing, thanks to Einstein's famous equation E=Mc^2). Dark energy is thought to account for 74 percent of the universe, while dark matter adds about 22 percent, and normal, visible matter contributes a puny 4 percent.

As if the discovery of dark energy weren't bizarre enough, it has stirred up a whole host other issues. For example, dark energy adds fuel to the fire of believers in multiple universes, or the idea that our own existence is just one of countless worlds in which the constants and conditions are different. There might be other universes in which dark energy doesn't exist, and the universe does slow in its expansion, cosmologists say. Maybe that's why our universe is so peculiar.