WASHINGTON—After a 70-year hunt, astronomers and physicists think they'll soon be able to identify "dark matter," a ghostly glue that keeps the galaxies, including our own Milky Way, from flying apart.
Telescopes in space and detectors buried deep underground are searching for this mysterious substance, which is at least five times more plentiful than ordinary matter, the stuff of stars, planets, trees and people.
Finding it would answer a fundamental question in science: What is much of the universe made of?
Dark matter is thought to consist of an enormous swarm of invisible particles left over from the Big Bang, the theoretical beginning of the universe 13.7 billion years ago.
These particles can't be seen through even the most powerful telescopes or microscopes. They can be detected only indirectly, by the way they interact with normal matter. Scientists are convinced that they exist because of the way their gravity affects the motion of visible galaxies.
"We owe our existence to dark matter," says the draft of a forthcoming report from the National Academy of Sciences, "Revealing the Hidden Nature of Space and Time." "Without the added gravitational attraction of dark matter, the stars and galaxies, including our own Milky Way, would likely never have formed."
"Dark matter holds together all structures in the universe—including our own Milky Way—and we still do not know what the dark matter is made of," said Michael Turner, assistant director for mathematics and the physical sciences at the National Science Foundation.
Five scientific teams in the United States, Canada, Britain and Europe are racing to be the first to identify dark matter.
"There is really great progress in this field, and there could be major discoveries soon," said David Cline, the leader of the Dark Matter Search Project at the University of California Los Angeles. "Underground lab experiments and space-based experiments are both capable of a discovery of dark matter in the next three years."
Dark matter is often paired with another, even stranger phenomenon known as dark energy. Discovered in 1998, dark energy is considered to be a weird repulsive force—sometimes called "negative gravity"—that drives galaxies apart at an ever increasing speed.
Between them, these shadowy twins account for 95 percent of the mass and energy of the universe, leaving only 5 percent for the familiar atoms such as hydrogen, carbon and iron.
"Dark matter played a crucial role in the past by causing galaxies to form, and dark energy will play a crucial role in the continuing evolution of the universe," the academy report says. "Understanding what dark matter and dark energy are are among the most compelling scientific questions of our time."
In a universe dominated by dark matter, galaxies are considered to be enormous spheres, called "halos," composed of invisible particles surrounding a much smaller visible disk of stars and planets, such as our Milky Way. The stars are like lanterns glowing in a dark forest.
Our galaxy is thought to contain at least 10 times more dark matter than ordinary matter. Astronomer Gerry Gilmore, at Cambridge University in England, reported that 7,000 galaxies he'd studied have 400 times more dark matter than ordinary matter.
British astronomers recently discovered a galaxy in the Virgo cluster, known as VIRGOHI21, that lacks any visible stars and consists almost entirely of dark matter plus a few wisps of hydrogen.
At this point, the leading candidate for dark matter is a "neutralino"—a theoretical particle, 100 times heavier than a hydrogen atom—that has never been detected. There are other possibilities, Cline said, but the odds right now favor the neutralino.
Neutralinos are also known as WIMPs, an acronym for "Weakly Interacting Massive Particle," because they don't interact strongly with ordinary atoms. They pass right through the Earth, which makes them extremely difficult to spot.
"A huge number of neutralinos are going through your body as we speak," Cline said in a telephone interview. "We know they're there."
To detect dark-matter particles, researchers use extremely sensitive thermometers—typically made of silicon, germanium or xenon—chilled almost to absolute zero. The detectors are in underground mines or mountain tunnels to avoid false signals from cosmic rays or other background radiation.
Once in a while, scientists predict, a neutralino or other particle will bounce off the nucleus of an atom in the detector. The impact will produce a tiny flash of heat that can be measured and analyzed.
This indirect method of detection has been likened to playing pool with an invisible cue ball. You can't see the cue ball, but you can see the other balls scatter across the table.
"It's a quarry worth hunting," said Daniel Akerib, a particle physicist at Case Western Reserve University in Cleveland. Akerib and his colleagues are about to start collecting data from a new detector half a mile below ground in an abandoned iron mine in northern Minnesota.
A more sensitive device is under construction in a nickel mine more than a mile deep in Sudbury, Ontario. The deepest detector lies almost two miles down in a salt mine in Yorkshire, England. Still others are in tunnels in Italy and France.
Space telescopes, meanwhile, are looking for evidence of huge masses of dark matter by the way they bend light from distant galaxies. Maps of galaxies trace the presence of dark matter by its effect on the motion of stars.
To pull these efforts together, the National Science Foundation and the Department of Energy have formed a Dark Matter Scientific Assessment Group, which will hold its first meeting June 29-30 in Washington.
For more information online about dark matter, go to:
(c) 2006, Knight Ridder/Tribune Information Services.
PHOTO (from KRT Photo Service, 202-383-6099): DARKMATTER
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