http://www.smh.com.au/news/science/cosmic-crash-leaves-dark-ring/2007/05/16/1178995195573.html
Cosmic crash leaves dark ring A ghostly ring of dark matter in a galaxy cluster, designated Cl 0024+17. Photo: Reuters/NASA Richard Macey May 16, 2007 - 8:33AM Shaped like a ring, it is invisible and 2.6 million light years across - as wide as two dozen Milky Way galaxies. But exactly what it is made of, no one knows. Astronomers using the Hubble space telescope have discovered a bizarre ring of dark matter, one of the most mysterious substances in the universe. Ordinary matter, from which planets, stars and people are made, makes up only about 4 per cent of the universe. About 70 per cent is believed to be dark energy, a sort of anti-gravity blamed for accelerating the expansion of the universe. The rest has been dubbed dark matter, because it is invisible and unknown. Although it should be all around us "nobody has ever found a lump of dark matter in the laboratory", Scott Croom, a University of Sydney astronomer said. Scientists think it must exist because the galaxies rotate so fast that they should fly apart. Dark matter is thought to be the unseen material holding them together. Now researchers from Johns Hopkins University, in Baltimore, say they have detected a ring of dark matter created when two clusters of galaxies collided long ago, 5 billion light years away. As they smashed together, dark matter inside fell to the centre of the impact, before sloshing back out. "The collision created a ripple of dark matter," said one Baltimore astronomer, M. James Jee. Although invisible, its gravity is so strong it distorted light from even more distant background galaxies, producing the ring effect detected by the Hubble telescope. Dr Jee compared it with watching ripples of water flowing over pebbles in a pond. "The pebbles' shapes appear to change as the ripples pass over them." Dr Croom described the discovery as "another piece of evidence that confirms the dark matter hypotheses" and said it could provide some clues to dark matter's nature. "The best suggestion so far is that it is some exotic elementary particle we don't know about." Dr Melanie Johnston-Hollitt, a University of Tasmania astronomer, said: "[Hubble's view is] like watching a head-on crash of two cars on a dusty road. You can imagine a ring of dust being thrown out in all directions. "Being able to study the way dark matter responds as it sloshes back and forth after a collision is really very important to our understanding of exactly what dark matter is." [Non-text portions of this message have been removed]
