In recent years, astronomers have found themselves faced with a nagging inventory problem. Received wisdom holds that dark matter and dark energy make up 95 percent of the universe, and ordinary matter, or baryons--the subatomic particles that form planets, stars and the like--account for the remainder. The problem is, the luminous matter detected with the aid of optical telescopes has amounted to a mere 10 percent of the expected ordinary matter, and the baryons inferred by other means bring that total to only 50 percent.

New findings are helping to bridge this gap between prediction and observation. In a paper published today in the journal Nature, scientists report having identified the probable source of the rest of this missing matter. Data from the Chandra X-ray Observatory, it appears, indicate that the lost baryons may be swimming in diffuse rivers of gas in the intergalactic medium too hot to see with an optical telescope.

Previous work had suggested that baryons might be inhabiting an infernally hot intergalactic gas, but the researchers did not know enough about the density of the baryons to draw firm conclusions about how many might be there. In the new study, Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics and his colleagues obtained high-quality spectra of the gas while it was illuminated by the flaring of the quasarlike galaxy Markarian 421 (see image). Based on those spectra, the team determined that the density of the baryons in the gas was sufficient to account for the missing matter.

But whether the region sampled in this study is representative of the rest of the universe is not known. "New ultraviolet and X-ray observatories are needed to complete the inventory of missing baryons," writes J. Michael Shull of the University of Colorado in an accompanying commentary. "But they will do much more, allowing astronomers to map out the cosmic web of filamentary intergalactic matter from which the first galaxies and stars were formed."