Almost 400,000 years after the universe was created in the big bang, it cooled sufficiently for neutral atoms to form, thereby allowing the pervasive light to propagate almost completely unhindered. Today, that light bathes the universe. We see it as the cosmic microwave background radiation (CMBR), and modern instruments study it intently to try to determine what happened in those early days of the cosmos. Among other things, it holds clues into how stars and galaxies subsequently formed and evolved. Galaxies tend to gather in clusters -- our own Milky Way galaxy, for example, and its local group of neighboring galaxies are at the edge of the Virgo Cluster. The intergalactic gas within clusters of galaxies is sometimes heated by shocks as it falls into the galaxies. That relatively dense, hot matter can scatter the primal light of the CMBR -- billions of years after the light was set free. Astronomers have been trying to search in maps of the CMBR for slightly fainter regions due to this effect, known as the Sunyaev-Zel'dovich Effect after the theorists who first proposed its existence in 1970.
SAO astronomer Niayesh Afshordi and three of his colleagues have just finished a new study of CMBR results using images from from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) satellite. They reach two dramatic conclusions. First, they report a convincing detection of the Sunyaev-Zel'dovich Effect based on examination of the regions around 193 clusters of galaxies known from X-ray observations to contain very hot gas. They use these results to make the first model-independent measurements of properties of this hot gas. Then the scientists completed a set of sophisticated computer simulations of the intracluster gas, and compared them to the observations. Most details fit together very well. However, to the astronomers' surprise, they found that to have a consistent result a significant amount of normal matter in the gas, about 35% percent, had to be missing. They speculate that some of this matter may have cooled and gone into forming some stars or even galaxies, but they note that there aren't enough stars or galaxiesto explain it all. About half of the missing, normal matter must reside in some still unknown phase of the vast reaches of intergalactic space. More research is needed to confirm these results and to resolve the new mystery.