"It's the most direct evidence that we have for dark matter," said Anthony Gonzalez, an assistant professor of astronomy at the University of Florida and a member of the team of astronomers who made the discovery. "You can actually see the separation between where the bulk of the matter is and the normal everyday matter."

These results are being published in an upcoming issue of The Astrophysical Journal Letters. The discovery was made with NASA's Chandra X-ray Observatory and other telescopes.

Despite considerable evidence for dark matter, some scientists have proposed alternative theories for gravity where it is stronger on intergalactic scales than predicted by Newton and Einstein, removing the need for dark matter. However, such theories cannot explain the observed effects of this collision.

"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 at Tucson, leader of the study. "These results prove that dark matter exists."

Gonzalez echoed Clowe, characterizing the results as raising a "significant challenge" to dark matter alternative theories.

In galaxy clusters, the "normal" matter, like the atoms that make up the stars, planets and everything on Earth, is primarily in the form of hot gas and stars. The mass of the hot gas between the galaxies is far greater than the mass of the stars in all of the galaxies. The galaxies and hot gas are bound in the cluster by the gravity of an even greater mass of dark matter. Without dark matter, which is invisible and currently can be detected only through its gravity, the fast-moving galaxies and the hot gas would quickly fly apart.

The team used about a week of Chandra time to observe the galaxy cluster 1E0657-556, which is also known as the "bullet cluster" because of a spectacular bullet-shaped cloud of extremely hot gas. The X-ray image shows that the bullet shape is due to a wind produced by the high-speed collision of a smaller cluster with a larger one.

Meanwhile, the Hubble Space Telescope, European Southern Observatory's Very Large Telescope and Magellan optical telescopes were used to determine the location of the mass in the clusters. This was done using a technique known as gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity.

Gonzalez assisted in the analysis of the Hubble Space Telescope images and otherwise contributed to the optical data analysis.

The hot gas in this collision was slowed by a drag force, similar to air resistance. 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 were the most massive component in the clusters, as proposed by alternative gravity theories, such a separation would not be seen. Instead, dark matter is required.

"This is the type of result that future theories will have to take into account," said Sean Carroll, a cosmologist who was not involved with the study. "As we move forward to understand the true nature of dark matter, this new result will be impossible to ignore."

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," said Clowe.

Other scientists involved in the research include Marusa Bradac of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC); Dennis Zaritsky of the University of Arizona's Steward Observatory; Maxim Markevitch, Scott Randall, Christine Jones and William Forman of the Harvard-Smithsonian Center for Astrophysics, Tim Schrabback of the University of Bonn, and Phil Marshall of KIPAC. Support for this work was provided by the National Science Foundation and NASA. This project was also partially supported by the Department of Energy through the Stanford Linear Accelerator Center.

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