It's possible that these hints are just a statistical fluctuation, and with more data, the excess will go away. It's also possible that Fermi's data really do show an excess of these photons, but that they're due to some artifact in the instrument — a systematic error.
"We've already gone through a lot of hypotheses about what could be wrong with the instrument, and all of them fail in some way or another," Finkbeiner said. "Something unlikely has happened here. Either a very unlikely statistical fluctuation, some kind of problem in the instrument that is masking itself in some unlikely way, or we have 130 GeV photons. They're all actually very unlikely, but one of them still happened."
"In my opinion, the most important issue is to rule out the possibility that the line feature in the data might be instrumental in origin," said Simona Murgia, an astrophysicist at the University of California, Irvine and a member of the Fermi collaboration galactic center analysis team. "Additional data from modified observations would help understand this better."
The situation is also complicated by a second, apparently unrelated, potential indication of dark matter in the Fermi data. In addition to 130-GeV photons, scientists have seen an excess of lower-energy gamma rays in the range of 2-3 GeV. While this signal is strong enough to rule out the chance that it's a statistical fluctuation, it could also be caused by regular astrophysical sources, such as pulsars.
But if the 130-GeV signal persists and can't be attributed to a systematic error, then astronomers may have found the first proof that dark matter exists, and a look at what it's made of.
"If it is a real line, it would be a 'smoking gun' of dark matter," said University of California, Irvine astrophysicist Kevork Abazajian, who's studied the other, lower-energy 2-3 GeVFermi gamma-ray signal. The proposed observing strategy would not shed much light on his feature, but it would help resolve the higher-energy signal, Abazajian said.
"They make a pretty compelling case," said Dan Hooper, an astronomer at the Fermi National Accelerator Laboratory in Batavia, Ill., and the University of Chicago who has also studied the lower-energy gamma-ray signal. "It would be great to have some more data from this direction of the sky, and the downsides of their proposed strategy seem minimal."
Hooper said he was skeptical that the signal Weniger and his team are chasing is actually dark matter, but that more data would help settle the matter.
Other projects are currently chasing after dark matter in different ways. The Alpha Magnetic Spectrometer (AMS), a particle detector attached to the outside of the International Space Station, is also looking for signs of dark-matter annihilation explosions in space. The first data from that experiment, announced in April, show a hint of evidence that could be caused by dark matter, but the findings are very preliminary. And if they do end up pointing toward dark matter, they suggest a different mass of WIMP from the Fermi results, so the two results aren't necessarily complementary.
Other experiments hope to catch dark-matter particles directly, on the very rare occasions they do collide with normal matter particles. Such detectors — which include the XENON Dark Matter Project in Italy, the LUX (Large Underground Xenon) experiment in South Dakota, and the SuperCDMS (Cryogenic Dark Matter Search) experiment in Minnesota — are buried deep underground, where almost nothing but dark matter can reach them. None has found definitive results so far.