Dark matter, the elusive stuff that makes up a quarter of the universe, has been seen in isolation for the first time. 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 by studying a galaxy cluster 3 billion light years away. "We had predicted the existence of dark matter for decades, but now we've seen it in action," said Bradac. "This is groundbreaking." Hot, luminous gas (red) separates from dark matter (blue) after the collision of two galaxy clusters about 3 billion light years away. (Image: NASA) Dark matter is fundamentally different from normal matter. It is invisible using modern telescopes because it gives off no light or heat, and it appears to interact with other matter only gravitationally. In contrast, luminous matter is everything commonly associated with the universe: the galaxies, stars, gas and planets. Past observations have shown that only a very small percentage of mass in the universe can be explained by regular matter. The new research is the first to detect luminous matter and dark matter independent of one another, with the luminous matter clumped together in one region and the dark matter clumped together in another. These observations demonstrate that there are two types of matter: one visible and one invisible. The results also support the theory that the universe contains 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 the University of Arizona's Doug Clowe, one of the study's key collaborators. "We believe these results prove that dark matter exists." The research is based on observations of a remarkable cosmic structure called the bullet cluster. This structure is actually 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.The separation of luminous gas appears red, and dark matter appears blue. (Image: Marusa Bradac, Kavli Institute for Particle Astrophysics and Cosmology) By measuring these distortions with the Hubble Space Telescope, the Magellan Telescopes and the Very Large Telescope, the team mapped out the location of all the mass in the bullet cluster. They then compared these measurements to x-ray images of the luminous matter taken with the Chandra X-ray Observatory and discovered four separate clumps of matter: two large clumps of dark matter speeding away from the collision, and two smaller 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. "These measurements are compelling," said KIPAC Director Roger Blandford. "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." The research will be published in Upcoming issues of the Astrophysical Journal and the Astrophysical Journal Letters. In addition to Bradac and Phil Marshall of KIPAC, team members include Douglas 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. Support was provided by the National Science Foundation and NASA; the project was also partially supported by the Department of Energy through SLAC.For more information, visit: www-group.slac.stanford.edu/kipac/