Dark matter is greatly contended in the scientific community. The laws of gravity put together by the theories of Sir Isaac Newton and Albert Einstein state that ordinary matter exerts specific gravitational forces, but those would be mathematically too weak to prevent galaxies from flying apart. Some scientists have argued that gravity remains stronger at distances greater than those put forth by Newton and Einstein (the theory of modified gravity), but others say it is more likely galaxies are held together by dark matter; the mysterious, invisible matter that would have a gravitational force six times stronger than regular matter.
The existence of dark matter has been proven, according to Doug Clowe of the University of Arizona in Tucson, Ariz., due to a 4,700-kilometer-per-second collision between two clusters of galaxies 100 million years ago that created an object referred to as 1E0657-556. Usually, dark matter cannot be observed independently because it is too well mixed with ordinary matter, but Clowe says 1E0657-556 reveals the gravitational influence of clouds of dark matter.
"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," Clowe said.
In galaxy clusters, most of the ordinary matter is observable as hot gas, but when the astronomers mapped out the hot gas using NASA's Chandra X-Ray Observatory, they found that the gravity field was not centered on the clouds. When the Hubble Space Telescope, the European Southern Observatory's Magellan telescope, and the Wide-Field Imager were used to measure the gravity field, it was found to bend light from distant background galaxies.
The two galaxies got their hot gas clouds entwined during the collision, but not their dark matter clouds. The electromagnetic forces between atoms usually create drag, which is why the hot gas clouds became tangled, but the force of gravity is the only thing that can cause dark matter to interact with other matter. This factor, Clowe said, was the evidence that supported dark matter's existence.
"You cannot explain the result using altered gravity," he said.
"This result proves beyond a reasonable doubt that there really is such as thing as dark matter," said Sean Carroll of the University of Chicago in a New Scientist interview. "The whole idea that modified gravity can do away with all dark matter is more or less ruled out," he said.
Carroll admits that modified gravity could still exist alongside dark matter, but the theory has been weakened by these results.
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