Once lost but now found. (Visible Matter).

Never mind about the whereabouts of dark matter, the mystery material that accounts for 95 percent of the mass of the universe. Astronomers haven't even been able to find all the visible matter--atoms and molecules--that they know should exist in nearby regions of the universe.

New observations confirm that most of the visible stuff lies hidden in vast, hard-to-detect gas clouds between galaxies. Over billions of years, the clouds have gathered into a spidery network of filaments connecting galaxies and galaxy clusters. Studies with NASA's Chandra X-ray Observatory suggest that the clouds contain twice as much visible matter as galaxies do.

Four independent teams of researchers used the beacons of X rays from distant quasars to probe the contents of several intergalactic clouds. En route to Earth, X rays from the quasars are absorbed by ionized oxygen and other ions that reside within the intergalactic clouds. The strength of the absorption reveals the temperature, density, and mass of a gas cloud. The researchers describe their work in several articles slated for the Astrophysical Journal.

The gas clouds range in temperature from 300,000[degrees] to 5 million[degrees]C. Ultraviolet detectors had previously revealed the coolest components of this gas (SN: 5/13/00, p. 310). But computer simulations had predicted and the new results have for the first time shown that most of the visible matter in clouds has higher temperatures and so can best be identified by X-ray detectors, says one of the articles' coauthors, Fabrizio Nicastro of the Harvard Center for Astrophysics in Cambridge, Mass.

"We have finally seen a large amount of the [visible] matter that had eluded detection before," he reports.

Several lines of evidence had shown that researchers had been missing most of the visible material. Calculations of the amount of hydrogen, helium, and a few other light elements forged just after the Big Bang indicated that there should be much more of this material in nearby reaches of the universe than had been found. Studies of gas clouds so distant that they reveal conditions in the early universe also provided evidence for a much higher amount of visible matter than astronomers had found locally.

The new studies "reveal that most of the visible matter in the universe is in the intergalactic medium," says theorist Jeremiah P. Ostriker of the University of Cambridge in England. He adds that the newly identified matter is at "exactly the temperature and density range" that he and Renyue P. Cen of Princeton University, as well as other researchers, had predicted (SN: 6/20/98, p. 390).

In an article to appear in Astronomy and Astrophysics, a team led by Luca Zappacosta of the University of Firenze in Italy describes a different technique for seeking the missing material. Instead of measuring the matter by detecting how much X-ray radiation it absorbs, the team detected X rays that the matter in gas clouds emits. This radiation also indicates that most visible matter lies hidden in the warm gas between galaxies.

Because visible matter traces the presence of the much more abundant but elusive dark matter, the new observations may also provide a more accurate map of dark matter, says Nicastro.