It’s too early to say for certain, but astronomers may have picked up a new clue to the nature of dark matter—invisible cosmic stuff with at least five times the mass of all the visible stars and galaxies combined.
The clue comes in the form of gamma rays, a kind of light the human eye can’t detect, emanating from a newly discovered dwarf galaxy called Reticulum 2. Reticulum 2, which hovers beyond the edge of the Milky Way, about 98,000 light-years from Earth, is fascinating in its own right: No more than a few thousand stars (compared with the Milky Way’s hundred billion or more) embedded in a clump of dark matter, it’s similar to the first tiny galaxies that appeared after the big bang. (See “Galaxy Hunters—The Search for Cosmic Dawn” to learn more about the earliest galaxies.)
The mystery of dark matter dates back all the way to the 1930s, when the legendary astronomer Fritz Zwicky first noticed that galaxies in clusters seemed to be moving under the gravity of some strange, unseen substance. But so far nobody has figured out what it really is. (To learn more about dark matter, see "A First Hint of the Hidden Cosmos" in National Geographic magazine.)
Reticulum 2, along with eight other recently discovered dwarf galaxies, are intriguing for what they can tell scientists about what the early universe might have looked like. But they’re also prime places to try and figure out what dark matter is, because they have more dark matter compared to visible matter than a full-size galaxy such as the Milky Way does.
Astrophysicists were especially interested to see if the galaxy was emitting gamma rays, because if dark matter is actually a type of still-undiscovered elementary particle, as many astrophysicists suspect, then the rays would be produced when the particles and their antiparticles meet and annihilate each other. So several teams began combing through data from NASA’s Fermi Gamma-ray Space Telescope, searching to see if there were any telltale signals coming from the direction of Reticulum 2.
Sure enough, one of the teams found the gamma rays it was looking for. “We saw them almost immediately,” says Matthew Walker, of Carnegie Mellon University, in Pittsburgh, a co-author of a paper on the discovery that has been submitted to the journal Physical Review Letters. If the team is right, it may have solved Zwicky’s age-old mystery.
If it’s right. A different team did its own analysis, and has declared in its own paper, which has been submitted to Astrophysical Journal Letters, that while there does seem to be an excess of gamma rays coming from Reticulum 2, it doesn’t rise to the level of significance. “Our group,” says lead author Alex Drlica-Wagner, of the Fermi National Accelerator Laboratory, which is formally affiliated with the Fermi telescope, “tends to be more conservative in our interpretations,” he says. “Outsiders tend to be bolder.”
Astronomers have claimed to see evidence of dark-matter particles in the past, only to have those claims shot down. But Drlica-Wagner doesn’t rule out the idea that further observations and analysis could convince the conservatives that the gamma rays are more significant than they now believe.
Sooner or later, astronomers are going to figure out what dark matter really is. There’s a chance it might be happening now.
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