A team of astronomers say they may have detected dark matter, the invisible substance thought to make up over 85 percent of all matter in the universe, for the first time in history.
The claim is controversial, and the findings, published in a new study in the Journal of Cosmology and Astroparticle Physics, will need to be borne out by further observations. But at least until it gets picked apart by other physicists, it’s one of the most exciting developments in the hunt for this omnipresent specter haunting the cosmos.
“This could be a crucial breakthrough in unraveling the nature of dark matter,” study author Tomonori Totani, an astronomer at the University of Tokyo, told The Guardian.
The ordinary “baryonic” matter that we see and touch, and makes up everything from planets to stars, doesn’t exist in great enough quantities to account for the formation of galaxies, because they simply don’t have enough mass to exert the gravitational pull to hold themselves together. Dark matter, outnumbering ordinary matter five-to-one, was to hypothesized to explain this discrepancy. Like a puppetmaster in the shadows, it pulls the stars and planets of galaxies together through its gravity, but doesn’t seem to interact with ordinary matter in any other way.
That, of course, makes its existence extremely difficult to demonstrate, and what dark matter actually is remains a mystery, even though it’s a cornerstone of modern cosmology. The more exotic theories range from so-called primordial black holes, which can be smaller than an atom and virtually impossible to see, to the echoes of parallel universes.
One of the prevailing ideas, though, is that dark matter is made up of particles called WIMPs, or weakly interacting massive particles. In addition to not interacting with light and ordinary matter, WIMPs are slower and heavier than baryonic particles, allowing them to clump together and form massive haloes. It’s in these haloes that galaxies form.
If WIMPs exist, then they should have anti-particles just like ordinary matter does, which we call antimatter. When WIMPs and their anti-particles collide, they should also annihilate each other and release energy in the form of gamma rays, and it’s these gamma ray emissions which scientists have been trying to detect for decades.
Gamma rays, however, are everywhere in the cosmos and given off by a host of sources, from supernovas to neutron stars. And so, to declare that gamma rays were emitted by annihilating dark matter, it must come from a region of space where there’s no other possible source that could produce them.
That, in a nutshell, is what the astronomers claim to have done using NASA’s Fermi Gamma-ray Space Telescope. In their study, the team analyzed fifteen years worth of data that the Fermi telescope took in an overlooked region near the center of the Milky Way, and found a halo of gamma rays that couldn’t be explained by its surroundings.
“We detected gamma rays with a photon energy of 20 gigaelectronvolts (or 20 billion electronvolts, an extremely large amount of energy) extending in a halolike structure toward the center of the Milky Way galaxy,” Totani said in a statement. “The gamma-ray emission component closely matches the shape expected from the dark matter halo.”
The intensity of the emissions, the astronomers found, matches the emissions predicted from the annihilation of WIMPs, suggesting that the particles have a mass around 500 times that of a proton.
“It turns out that dark matter is a new particle not included in the current standard model of particle physics,” Totani added. “This signifies a major development in astronomy and physics.”
Some of Totani’s colleagues, however, are skeptical.
“I appreciate the author’s hard work and dedication, but we need extraordinary evidence for an extraordinary claim,” Kinwah Wu, a theoretical astrophysicist at University College London, told The Guardian. “This analysis has not reached this status yet. It is a piece of work which serves as an encouragement for the workers in the field to keep on pressing.”
Totani is optimistic, but is aware that the detection isn’t clear-cut yet. One way to substantiate to the claim, he said, would be to detect the same gamma-ray signature in small dwarf galaxies that orbit the Milky Way. “This may be achieved once more data is accumulated, and if so, it would provide even stronger evidence that the gamma rays originate from dark matter.”
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