"Proof of dark matter"
Scientists using Nasa's Chandra X-ray observatory and other telescopes claim to have found direct evidence for the existence of dark matter.
In astrophysics, dark matter refers to material which cannot be seen but is thought to exist through its gravitational effects on visible matter. Its composition is unknown, but is thought to consist of elementary particles, dwarf stars and non luminous gas.
Some researchers have proposed alternative theories for dark matter, but the scientists behind the current study into a "tremendous" collision of two large clusters of galaxies believe that only the existence of dark matter can explain its effects.
During the collision, which Maxim Markevitch of the Harvard-Smithsonian Centre for Astrophysics described as "the most energetic cosmic event, besides the big bang, which we know about," the scientists argue that dark matter and normal matter were wrenched apart.
After observing the galaxy cluster 1E0657-56, which is also known as the bullet cluster, the scientists were able to determine the mass in the clusters. A separation between dark and normal matter was observed through the hot gas in the collision being slowed while the dark matter remained the same.
"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said lead researcher Doug Clowe of the University of Arizona at Tucson. "These results are direct proof that dark matter exists."