Astronomers find first direct evidence of dark matter

A U.S. university's research team has found first direct evidence for the existence of dark matter with NASA's Chandra X-ray Observatory and other telescopes.

Astronomers have known since the 1930s that most of the universe must be made up of something other than normal matter, which makes stars, planets, all things and creatures. It has been theorized that the universe must contain about five times more dark matter than normal matter given the way that galaxies move through space and scientists' understanding of gravity.

But for the past 70 years, no one had any direct persuasive evidence that dark matter even exists, until a research team at the University of Arizona observed a galaxy cluster named 1E0657-56 with the Chandra telescope.

The cluster is also known as the bullet cluster, because it contains a spectacular bullet-shaped cloud of hundred-million-degree gas. The X-ray image produced by Chandra showed the bullet shape was due to a wind produced by the high-speed collision of a smaller cluster with a larger one.

Observations showed the bulk of ordinary matter is in the hot gas clouds left in the wake of the galaxies. Part of this million-degree plasma of hydrogen and helium, the part from the smaller cluster, form a spectacular bullet-shaped cloud because a bow shock, or supersonic shock wave, is created in the collision of nearly 20 million km per hour.

The astronomers also used the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes to determine the location of the mass in the clusters by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity.

The hot gas in this collision was slowed by a drag force similar to air resistance, the astronomer found. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity.

This produced the separation of the dark and normal matter seen in the data. If hot gas was the most massive component in the clusters as proposed by alternative gravity theories, such a separation would not have been seen. Instead, dark matter is required.

"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 Doug Clowe of the University of Arizona who led the study, "These results are direct proof that dark matter exists."

This result also gives scientists more confidence that the Newtonian gravity familiar on Earth and in the solar system also works on the huge scales of galaxy clusters.

"We've closed this loophole about gravity, and we've come closer than ever to seeing this invisible matter," Clowe said.

Source: Xinhua