Dark Matter Takes a Solo

http://sciencenow.sciencemag.org/cgi/content/full/2007/515/3?rss=1
"Astronomers have spotted a ring of elusive dark matter devoid of stars, galaxies, and gas. The discovery gives astrophysicists more evidence to counter skeptics who doubt the existence of dark matter. Most astronomers and astrophysicists believe that some form of mysterious dark matter holds galaxies and clusters of galaxies together. However, dark matter interacts with ordinary matter so weakly that it hasn't yet been directly detected. A few researchers argue that dark matter doesn't exist and galaxies hang together because gravity behaves differently than generally believed over vast distances.
These so-called modified gravity theories suffered a blow last year when astronomers spotted a collision between two galaxy clusters in the Bullet Cluster (ScienceNOW, 21 August 2006). The smash-up stripped the gas from the clusters, leaving a large blob of ordinary gas glowing between two clumps of galaxies. From its brightness, the researchers could tell the gas contained more matter than the galaxies. Yet researchers could tell that gravity was stronger near the galaxies by the way it distorted the images of more-distant galaxies. That paradox can be explained easily if the galaxies are embedded in two enormous globs of dark matter that supply the light-bending gravity. Many researchers say the formation can't be explained by modified gravity theories.
Now, a team led by Myungkook Jee of Johns Hopkins University in Baltimore, Maryland, has spotted an even more convincing case. Using the Hubble Space Telescope, the team spotted another collision of clusters, this one some 5 billion light years away. Whereas Earth-bound observers see the collision in the Bullet Cluster from the side, the Hubble sees the new collision along the line of movement, as if standing on a train track watching two locomotives crash head-on. This perspective allowed the team to observe a ring of distortions around the central cluster, suggesting that the collision splattered out a circle of dark matter almost empty of ordinary matter, as they report in a paper to be published in the Astrophysical Journal. "It's the first time that dark matter has been detected in its own structure," Jee says.
If the new observation holds up, it will close a final loophole in the Bullet Cluster analysis, says Douglas Clowe, an astronomer at Ohio University in Athens. That earlier observation could be explained if a weird force existed that pulled only on matter in galaxies, and not between the atoms in the gas. If so, that force, instead of gravity from dark matter, could be bending the light passing through clumps of galaxies. But the ring of distortions in the new cluster contains no galaxies, Clowe says, so such a force cannot explain the observed distortions. Clowe also notes, however, that Jee and colleagues had to "dig down into the noise" to extract the image. "It's certainly very suggestive that something is going on, but I don't think that the data they've presented in the paper is conclusive.""