A team of astronomers has revealed that the Hubble Space Telescope has provided the best evidence yet for the existence of dark matter in the form of a "ghostly ring" around a galaxy cluster.
The ring formed long ago during a collision between two huge galaxy clusters. Astronomers have a head-on view of the collision because it occurred along Earth's line of sight. From this perspective, the dark-matter structure looks like a ring.
The team said that its discovery, due to be published on 1 June in the Astrophysical Journal, represents the first record of dark matter distribution that differs substantially from the distribution of ordinary matter.
The ZwCl0024+1652 cluster is located five billion light years away from Earth. The ring measures 2.6 million light-years across, and was found unexpectedly while the team was mapping the distribution of dark matter within the cluster.
Astronomers have long suspected invisible dark matter to be the source of additional gravity that holds galaxy clusters together. If galaxy clusters relied only on the gravity from their visible stars as the binding force, they would fly apart.
Although what actually constitutes dark matter is unknown, one hypothesis is that it comprises some type of elementary particle that pervades the universe.
"This is the first time we have detected dark matter with a unique structure, different from that of the gas and galaxies in the cluster," said M. James Jee, of Johns Hopkins University, a member of the team that spotted the dark matter ring.
"Although the invisible matter has been found before in other galaxy cluste rs, dark matter has never been detected to be so largely separated from the hot gas and the galaxies that make up galaxy clusters.
"By seeing a dark matter structure that is not traced by galaxies and hot gas, we can study how differently it behaves from normal matter."
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