for National Geographic News
An international team of astronomers has mapped what appears to be a ring of dark matter around a massive galaxy cluster located some five billion light-years from Earth.
"We believe that this is the strongest evidence yet for the existence of dark matter," James Jee of Johns Hopkins University in Baltimore, Maryland, said today in a telephone briefing with reporters.
Dark matter is a mysterious substance that scientists believe makes up most of the matter in the universe. But because it neither emits nor reflects light, its existence is inferred through indirect means. (See a computer-simulated picture of dark matter.)
The new finding adds to a growing body of evidence that dark matter exists.
Last August another team of scientists said that they detected dark matter in a galaxy cluster called the bullet cluster.
The new evidence comes from a galaxy cluster called ZwC10024+1652. The Hubble Space Telescope took an image of the cluster in 2004.
Astronomers mapped the distribution of dark matter through observations of how gravity bends the light of more distant background galaxies in the image. This technique is called gravitational lensing.
Jee explained that while the dark matter itself is invisible, it distorts the light coming from the background galaxies somewhat, like a ripple of water passing over pebbles at the bottom of a pond.
As the astronomers made the map, they noticed the dark matter formed a ghostly ring around the galaxy cluster. The ring measures 2.6 million light-years across.
The map also shows that dark matter is distributed within the cluster.
Richard White, a team member from the Space Telescope Science Institute in Baltimore, explained at the briefing that the technique allows scientists to determine the distribution of all the matter in the galaxy cluster.
"We can have great confidence that that ring is there and that it really is quite a different distribution than the rest of the matter of the cluster," he said.
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