DECADES OF THEORY AND OBSERVATION have forced astronomers to the improbable conclusion that the universe is filled with an exotic form of matter invisible to human eyes and to advanced particle detectors . Now, finally, many scientists working with the Chandra X-ray Observatory have rendered this notion much more tangible by making precise, albeit indirect, measurements of so-called dark matter hovering around a galaxy group 4 billion light-years away.

One of the most fruitful places to hunt down dark matter is in the clouds of hot X-ray-emitting gas that fill large clusters of galaxies. The energy of the emission depends on the gravitational forces acting on the gas, which in turn depend on the cluster’s total mass. In this way, John Arabadjis and Mark Bautz of the Massachusetts Institute of Technology, along with a colleague at Penn State, used Chandra to weigh a galaxy cluster in the constellation Draco. Stars, gas, and dust can account for about 20 percent of the measured mass; the rest must be dark matter.

Chandra’s high-resolution optics allowed Arabadjis and Bautz to make a detailed map of all gravitating material within the cluster . The map shows that dark matter is more common in the center of the cluster, and it offers the best look yet at how dark particles dump together. Future Chandra observations may clarify whether more than gravity binds the particles of dark matter. “There are so many different explanations for what dark matter could be. Right now we know so little,” says Bautz.

                                                                                             —Maia Weinstock



December 2001. (Pg. 14)

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