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Gravitational Lensing Explanation
This illustration explains how gravitational lensing, a prediction of Einstein's theory of general relativity, can be used to determine the location of mass in a galaxy cluster. Gravity from mass in the galaxy cluster distorts light from background galaxies. In the idealized case shown here, two distorted images of one background galaxy are seen above and below the real location of the galaxy. By looking at the shapes of many different background galaxies, it is possible to make a map showing where the gravity and therefore the mass in the cluster is located. This is an excellent technique for studying dark matter.
Scale: Image is 13.5 x 10.6 arcmin
(Illustration: NASA/CXC/M.Weiss)

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	X-ray/Optical/Lensing Map Composites of 1E 0657-56 These images show the galaxy cluster 1E 0657-56, also known as the bullet cluster. The optical image from Magellan and HST shows galaxies in orange and white. Hot gas in the cluster, which contains the bulk of the normal matter in the cluster, is shown by the Chandra X-ray Observatory image in pink. Most of the mass in the cluster is shown in blue, as measured by gravitational lensing, the distortion of background images by mass in the cluster. This mass is dominated by dark matter. The clear separation between normal matter and dark matter has not been seen before and gives the strongest evidence yet that most of the matter in the Universe is dark. View Motion Graphic Scale: Images are 7.5 x 5.4 arcmin (Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.)

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	Chandra X-ray Image of 1E 0657-56 Dark matter and normal matter have been wrenched apart by the tremendous collision of two large clusters of galaxies. Never previously seen, this discovery, made with NASA's Chandra X-ray Observatory and other telescopes, gives direct evidence for the existence of dark matter. Scale: Full-field image is 13.5 x 10.6 arcmin (Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.)

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Galaxy Cluster in Perspective
This optical image from Hubble and Magellan shows a close-up (inset) of one of the galaxies, a spiral galaxy approximately the same size as the Milky Way, within the galaxy cluster known as 1E 0657-56. The full-field view shows over a thousand galaxies in this cluster. These immense objects are among the largest structures in the Universe.
View Motion Graphic
Scale: Full-field image is 7.5 x 5.4 arcmin
(Credit: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.)

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4-Panel Illustrations of Cluster Collision
These stills show four stages from an artist's representation of the huge collision that is taking place in the bullet cluster. Hot gas, containing most of the normal matter in the cluster, is shown in red and dark matter is shown in blue. During the collision the hot gas in each cluster is slowed and distorted by a drag force, similar to air resistance. A bullet-shaped cloud of gas forms in one of the clusters. In contrast, the dark matter is not slowed by the impact because it does not interact directly with itself or the gas except through gravity. Therefore, the dark matter clumps from the two clusters move ahead of the hot gas, producing the separation of the dark and normal matter seen in the image.
View Animation
(Illustrations: NASA/CXC/M. Weiss)

A better explanation;

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 is fundamentally different from luminous matter, which makes up only 4 percent of the mass of the universe. It 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—galaxies, 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.