Astronomers find evidence of missing matter.

If astronomers were in charge of a lost-and-found department, they would have been fired long ago. Most have come to terms with the notion that at least 90 percent of the matter in the universe must consist of some strange, dark material that they cannot directly detect. More embarrassingly, astronomers had lost track of most of the baryons--ordinary, visible matter made of protons, electrons, and neutrons.

In the standard picture of cosmic evolution, so-called dark matter provided the scaffolding upon which the baryons collected. During the first several billion years of cosmic history, the baryons--mostly hydrogen--formed vast gas clouds. Some ended up in galaxies and galaxy clusters, but that accounts for less than half the baryons originally in the clouds.

New observations suggest that astronomers may have found the missing material just where theorists predicted: in intergalactic space. Over billions of years, the vast clouds of hydrogen gas condensed into a spidery network of filaments connecting galaxies and galaxy clusters.

Stripped of its electrons, the hydrogen in the filaments can't radiate light and isn't easily detected. Instead, the Hubble Space Telescope found evidence of that hydrogen by searching for highly ionized oxygen. Forged at the core of stars and dumped into space when the stars died an explosive death, the ionized oxygen associated with the ionized hydrogen.

To look for the oxygen, Todd M. Tripp and Edward B. Jenkins of Princeton University and Blair D. Savage of the University of Wisconsin-Madison studied light from a distant quasar that slices through billions of light-years of space. The ionized oxygen absorbs specific wavelengths of light from the quasar beacon. Tripp's team used the Space Telescope Imaging Spectrograph on Hubble to find the characteristic fingerprints.

Looking at the quasar QSO H1821+643, the researchers detected four fingerprints that serve as evidence of relic gas clouds, they report in the May 1 ASTRO-PHYSICAL JOURNAL LETTERS. With the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, Tripp and other colleagues found an additional absorption feature, they note in an upcoming issue of the same journal.

Although Tripp emphasizes that he's working with small numbers, he says the remnant clouds that his team has found indicate that relatively nearby reaches of intergalactic space could hold about half the missing hydrogen. Other studies with FUSE and Hubble corroborate the finding, he notes.

The observations, says Jeremiah P. Ostriker of Princeton, confirm the results of computer simulations that he and Princeton colleague Renyue Cen had previously developed (SN: 6/20/98, p. 390). According to their model, half of all cosmic baryons are now floating in intergalactic space at temperatures between 100,000 and several million kelvins.

Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Md., says that he's now calculating whether NASA's Chandra X-Ray Observatory can record the faint emission from gases in that temperature range. If Chandra succeeds, astronomers will have more confidence that they've finally located some of the missing matter that roams the universe.