Dark energy has shaken the fields of physics and astronomy, much as Copernicus did five centuries ago when he declared that the Earth revolved around the sun.
Ten years later, the astronomers who made that claim say their findings have been confirmed repeatedly and made more precise. But they confess that no one -- including them -- understands what this mysterious dark energy is.
"We don't know any more today than we did 10 years ago," Saul Perlmutter, the leader of one of the discovery teams, said last month at a conference sponsored by NASA.
Mario Livio, a theorist at the Space Telescope Science Institute, a NASA affiliate in Baltimore, said it was shocking to realize that "we don't have an explanation for 74% of everything there is."
""It's as if we had no idea what water is," Livio said, "even though water covers three-quarters of the Earth."
The National Academies, the nation's premier scientific organization, says that solving this mystery should be astronomers' highest priority. Earlier this month, the Academies' National Research Council urged NASA and the Department of Energy to seek funds for what it calls a Joint Dark Energy Mission in 2009.
The evidence for dark energy came from observations that the universe is expanding at an accelerating rate, not slowing down as the law of gravity would seem to dictate.
In fact, astronomers say, gravity had been slowing the expansion of the universe for more than half its life, since its birth in the theoretical big bang 13.7 billion years ago.
But in the last 5 billion years, dark energy -- a sort of negative gravity or repulsive force -- has overcome gravity and is driving galaxies apart at an ever-increasing rate.
"The universe was slowing down; now it's speeding up," said Perlmutter, an astrophysicist at the University of California in Berkeley.
A cosmic contest
Adam Riess, the leader of the rival discovery team at the Space Telescope Science Institute in Baltimore, called the contest between gravity and dark energy a "cosmic tug of war."
"Today, dark energy is winning that battle," Riess said. He likened gravity to a brake on the expanding universe, and dark energy to an accelerator.
Perlmutter and Riess made their discovery by using the Hubble Space Telescope to measure the distance to brilliant exploding stars called supernovae.
Since then, Hubble and other telescopes have tracked 25 more supernovae in galaxies at various distances from Earth. By observing how fast the galaxies are moving, scientists can determine the expansion rate of the universe.
The measurements showed that the universe "is now expanding about 20% faster than 5 billion years ago," Riess said.
New ground and space telescopes are coming on line that could shed light on the nature of dark energy and perhaps help solve another mystery, so-called dark matter. Dark matter, which makes up about a quarter of the stuff in the universe, is thought to consist of tiny, unseen particles that haven't been identified.
Scientists say about 74% of the universe is made of dark energy, 22% of dark matter and 4% of ordinary matter: the stuff of stars and people and iPods.