Seeing Darkness
Dark side of Milky Way
Fermi Gamma-ray Space Telescope may have spotted signs of the annihilation of dark matter particles at the core of the Milky Way, as shown here in an artist's illustration
For years, most claims that scientists had found evidence of dark matter, the ghostly material believed to account for more than 80 percent of the universe's mass, have seemed to dissolve into thin air. But a new claim of dark matter detection may have more than a dollop of cosmic credibility, scientists say. Physicists Dan Hooper of the Fermi National Accelerator Laboratory in Batavia, Ill., and the University of Chicago and Lisa Goodenough of New York University base their findings, posted October 15 at arXiv.org, on an unexplained excess of energetic gamma rays emitted from the core of the galaxy. The gamma rays were recorded over the past two years with an instrument aboard NASA's Fermi Gamma-ray Space Telescope, launched in 2008. Dark matter, like ordinary atomic matter, is expected to concentrate at the galaxy's center. That makes the Milky Way's crowded core one of the most promising places to look for signs of the dark stuff, theorists agree. It's also one of the most complex places to search, because the core is riddled with a variety of ordinary but poorly understood sources of gamma ray emission, notes Fermi scientist Steve Ritz of the University of California, Santa Cruz. Hooper and Goodenough analyzed gamma rays recorded by Fermi from the innermost 175 light-years of the galaxy and found a sharply rising gamma-ray signal that peaked at energies between 2 billion and 4 billion electron volts, about a billion times the energy of visible light. Hooper asserts that the location and energy of the gamma rays can't easily be explained by run-of the-mill sources, such as ultradense, rapidly spinning stars called pulsars. "In our paper, we discussed a number of astrophysical possibilities for the origin of the signal, including a population of pulsars, cosmic ray interactions and emission from our galaxy's supermassive black hole," notes Hooper. "And in the end, no combination of any astrophysical sources could give us the signal we're seeing," he adds. "Eventually we just got fed up and concluded there doesn't seem to be a way to explain the signal except for one thing we tried dark matter and it fit beautifully without any special bells or whistles."
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