WASHINGTON — Ten years ago, an unexpected astronomical discovery stunned the scientific world: Two rival teams of astrophysicists separately claimed that most of the universe is made of an invisible substance they called ``dark energy.'' Only a tiny fraction, they said, consists of the ordinary atoms that make up stars, chairs and people.
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, not the other way around.
Ten years later, the astronomers who made that astonishing 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 at an anniversary conference this month 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 percent of everything there is.''
``We need humility,'' Livio said. ``It's as if we had no idea what water is, 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 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.
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 original 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 percent faster than 5 billion years ago,'' Riess said. ``Hubble confirmed that we're on the right track.''
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 enormous numbers of tiny, unseen particles that haven't been identified.
Scientists now think that about 74 percent of the universe is made of dark energy, 22 percent of dark matter and only 4 percent of ordinary matter.
Although astronomers don't know what dark energy is, theorists have proposed various suggestions.
``We have a number of ideas, but no clue which one is right,'' Livio said. ``It's like we're investigating a crime scene. We've got some suspects. We have some clues. But we're not ready to convict anybody.''
ON THE WEB
More information on dark energy.