Dark matter is one of the universe's most befuddling, and elusive, components. It could make up roughly a quarter of the universe's total mass and energy, yet no one knows for sure because no one has actually seen it. Well, it may be showing itself at last. nasa's Fermi Gamma-ray Space Telescope has recorded high-energy gamma-ray light emanating from the center of the Milky Way that fits well with dark matter predictions. “I would consider it currently the most exciting signal that we have,” said physicist Rafael Lang of Purdue University, who was not involved in the study, at the American Physical Society's meeting in April in Savannah, Ga. If the light were truly caused by dark matter, it would be the first indirect detection of the particles that make up this shadowy substance.
Dark matter is most likely made of so-called weakly interacting massive particles (WIMPs), particles that would be their own antimatter partners and therefore destroy one another on colliding. Such WIMP annihilations would produce normal matter particles that would in turn create gamma-ray photons. Because dark matter should be densest at the Milky Way's core, that is the best place to look for that light.
Scientists had previous hints that the Fermi telescope is seeing more light at the galactic center than expected. [For news of another unexplained Fermi signal, see page 42.] But where earlier analyses were inconclusive, the new study found a stark signal: an excess of gamma-ray photons extending at least 5,000 light-years from the Milky Way's center (above). “It looks exactly like we've always expected dark matter to look,” says Dan Hooper of Fermi National Accelerator Laboratory in Batavia, Ill., an author of the study.
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Of course, extraordinary claims require extraordinary evidence. Most scientists are reserving judgment until the signal is seen by other instruments or in other places. But dark matter may have gotten a little less elusive.