WASHINGTON (AP) _ Astronomers have detected tendrils of hydrogen in the vast dark between galaxies _ mysterious matter that accounts for about 5 percent of the universe, NASA said Wednesday.

Astronomers believe at least 90 percent of the matter in the universe is hidden in an exotic dark form that hasn't yet been seen directly.

The other 10 percent consists of baryonic, or ordinary, matter _ everything from stars to skyscrapers to people. And it has been embarrassing to scientists that, until now, they have been able to find only half of this ordinary matter.

Now, using the Hubble Space Telescope, scientists have uncovered the missing half, shedding new light on the large-scale structure of the universe and confirming previous predictions of how much hydrogen was manufactured in the first few minutes of the universe's birth in the Big Bang.

``This is a successful, fundamental test of cosmological models,'' said Todd Tripp, a Princeton University researcher who worked to find the missing hydrogen. ``This provides strong evidence that the models are on the right track.''

The results of Tripp and his collaborators are being published in the May 1 issue of the Astrophysical Journal Letters.

The astronomers think the missing matter exists as highly charged hydrogen between galaxies, but since such hydrogen is hard to see, they had to seek indirect evidence. In space, the hot hydrogen heats oxygen into an excited state that can be observed. The oxygen was probably spewed into space by exploding stars.

Astronomers found the oxygen by using the light of a distant quasar to probe the invisible space between the galaxies, like shining a flashlight beam through a fog. Quasars are distant, very energetic, stellar objects that can spew X-rays and visible light equal to the total brightness of trillions of stars.

The presence of invisible matter was determined years ago by astronomers who measured the motion of stars within galaxies. They determined that stars and clouds that shine and can be seen from Earth did not contain enough mass to hold the galaxies together.

Thus, there had to be other matter to provide the gravitational force that keeps the galaxies from flying apart.

Since then, astronomers have been scrambling to try to find and identify this missing matter because it has profound implications about the motion and ultimate destiny of the universe.


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