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August 21, 2006

Scientists observe dark matter in isolation for the first time

By Kelen Tuttle

Scientists have observed dark matter, the elusive stuff that makes up a quarter of the universe, in isolation for the first time. By studying a galaxy cluster 3 billion light years away, Marusa Bradac of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), located at the Department of Energy's Stanford Linear Accelerator Center (SLAC), and her colleagues made the landmark observations, which were announced today at a NASA teleconference.

"We had predicted the existence of dark matter for decades, but now we've seen it in action," said Bradac. "This is groundbreaking."

Said KIPAC Director Roger Blandford: "These measurements are compelling. The direct demonstration that dark matter has the properties inferred on the basis of indirect arguments shows that we are on the right track in our quest to understand the structure of the universe."

Dark matter, which makes up about 26 percent of the mass of the universe, is fundamentally different from luminous matter, which makes up only 4 percent. The remaining 70 percent of the cosmos is dark energy. Dark matter is invisible to modern telescopes because it gives off no light or heat, and it appears to interact with other matter only gravitationally. In contrast, luminous matter makes up everything commonly associated with the universe¬ógalaxies, stars, gases and planets.

Past observations have shown that luminous matter explains only a very small percentage of mass in the universe. The new research is the first to detect luminous matter and dark matter independent of one another, with the luminous matter clumped in one region and the dark matter clumped in another. These observations demonstrate the existence of two types of matter: one visible and one invisible.

The results also support the theory that the universe contains about five times more dark matter than luminous 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," said study collaborator Douglas Clowe of the University of Arizona. "We believe these results prove that dark matter exists."

The research is based on observations of a remarkable cosmic structure called the bullet cluster, which consists of two clusters of galaxies passing through one another. As the two clusters cross at a speed of 10 million miles per hour, the luminous matter in each cluster interacts with the luminous matter in the other cluster and slows down. But the dark matter in each cluster does not interact at all, passing right through without disruption. This difference in interaction causes the dark matter to sail ahead of the luminous matter, separating each cluster into two components: dark matter in the lead and luminous matter lagging behind.

To detect this separation of dark and luminous matter, researchers compared X-ray images of the luminous matter with measurements of the cluster's total mass. To learn the total mass, they took measurements of a phenomenon called gravitational lensing, which occurs when the cluster's gravity distorts light from background galaxies. The greater the distortion, the more massive the cluster.

By measuring these distortions using the Hubble Space Telescope and the ground-based Magellan Telescopes and Very Large Telescope, both in Chile, the team mapped out the location of all the mass in the bullet cluster. The scientists then compared these measurements to X-ray images of the luminous matter taken with NASA's Chandra X-ray Observatory and discovered clumps of dark matter speeding away from the collision and clumps of luminous matter trailing in their wake. The spatial separation of the clumps proves that two types of matter exist, while the extreme difference in their behavior shows the exotic nature of dark matter.

This research will be published in forthcoming issues of the Astrophysical Journal and the Astrophysical Journal Letters. Team members include Bradac and Phil Marshall of KIPAC; Clowe and Dennis Zaritsky of the University of Arizona's Steward Observatory; Anthony Gonzalez of the University of Florida; Maxim Markevitch, Scott Randall, Christine Jones and William Forman of the Harvard-Smithsonian Center for Astrophysics; and Tim Schrabback of the University of Bonn. The National Science Foundation and NASA supported the work.

Kelen Tuttle is a SLAC science writer.

Editor Note:

Images of dark matter are available on the web at



Neil Calder, Stanford Linear Accelerator Center: (650) 926-8707,


Marusa Bradac, Kavli Institute for Particle Astrophysics and Cosmology: (650) 926-2639,

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