3. Cosmic Collision Brings Dark Matter Into View
A violent collision between a pair of massive galaxy clusters 3 billion light-years from Earth has turned up the most direct evidence yet of dark matter, the invisible substance thought to make up 80 percent of the matter in the universe. To catch dark matter in action, astrophysicists at Stanford University's Kavli Institute and the universities of Arizona and Florida compared X-ray images of visible matter in the clusters with a map of the total mass, which they obtained by measuring the degree to which light from distant galaxies was bent as it passed by, a phenomenon known as gravitational lensing.
When the researchers compared the two images, "the two didn't line up," says Kavli Institute astrophysicist Marusa Bradac. "This tells us there must be something there, and that it is dark matter." Because dark matter doesn't interact with regular matter, or with itself, it passes right through everything. So when the two galaxy clusters smashed together at a staggering 10 million miles per hour, visible matter slowed down in the crush of the collision, but dark matter flew through unfettered. "The visible matter created a kind of traffic jam in the middle, whereas dark matter has its own highway," Bradac says. The different locations of the two kinds of matter are seen in the image, where red represents visible matter and blue represents the dark stuff.
If dark matter exists, it should be present on Earth too. "There are dark matter particles around me and you right now," says Bradac. The next big step for physicists is to detect it in a terrestrial laboratory. That effort got a major boost two months after the Kavli crew announced its discovery, when the most sensitive — and recently upgraded — dark matter detector in the world went online. The Cryogenic Dark Matter Search (CDMS), buried half a mile deep in an old Minnesota iron mine to shield it from cosmic rays, searches for collisions between dark-matter particles called WIMPS and ordinary atoms in 19 hockey-puck-size hunks of germanium. The apparatus is tuned to spot heat energy deposited by these rare impacts, known as scattering events, so the temperature of the instrument's underground silicon and germanium detectors is held at a chilly –459.58 degrees Fahrenheit, just above absolute zero.
Physicists could detect dark matter in the next five years, says Bradac, assuming it has the properties scientists expect. CDMS physicist Dan Akerib of Case Western Reserve University is wagering that it does. "If WIMPS are just around the corner, we might be able to see something," he says. "I'm betting 10 years of my life that they are."
