Gamma-ray image of the Milky Way’s halo.
Astronomers have detected a glow that they believe could be the first sign of dark matter. If confirmed, this could represent a historic step towards identifying one of the greatest enigmas in cosmology.
A still unexplained glow of gamma rays detected in the outer regions of the Milky Way may provide the first direct clue as to what dark matter is made of, one of the greatest enigmas in modern cosmology.
Dark matter is thought to represent around 85% of all mass in the Universe, but the particles that make it up have never been directly detected. We know that it exists mainly due to its gravitational effects on the rotation of galaxies and the way it bends light.
One of the main candidates to explain this invisible component are the so-called weakly interacting massive particles, known by the English acronym WIMP. These particles, if they exist, practically do not interact with normal matter, which makes their detection extremely difficult. But theoretical models suggest that, every now and then, two WIMPs can self-destruct each other, producing a small “flash” of high-energy radiation in the form of gamma rays, explains .
If dark matter is indeed distributed throughout the galaxy and is formed by WIMPs, the sky should show a diffuse glow of gamma rays resulting from these self-annihilations. Astronomers have been discussing for more than a decade whether an excess of gamma rays observed at the center of the Milky Way could be such a signature, but so far analyzes have not been conclusive.
Now, Tomonori Totanifrom the University of Tokyo, claims in a study this Tuesday that he found a similar signal, not in the center of the galaxy, but in its outer halo.
The researcher analyzed 15 years of data from NASA’s Fermi Gamma-Ray Space Telescope, focusing on the peripheral areas of the Milky Way. Totani began by building a detailed model of all the gamma-ray radiation that would be expected in that region, taking into account known sources: stars, cosmic rays and large bubbles of radiation observed above and below the galactic disk. Then, he subtracted this contribution from the radiation actually measured by Fermi.
The result was a residual glow of gamma rays, with energies around 20 gigaelectron volts — a value that fits well within the range of energies expected from self-annihilating WIMP particles, maintains the researcher, who considers this signal “the most promising dark matter radiation candidate known to date”.
If even Totani reacted with skepticism to the results he obtained, other experts also call for caution. Francesca Calore, from the National Center for Scientific Research in Annecy, France, considers that the result deserves in-depth study, but warns that it is “premature” already associate it with dark matter. Accurately modeling all the “normal” sources of gamma rays in the Milky Way is extremely difficult, and the models used have not been tested to the limit.
Silvia Manconi, from the Sorbonne, shares the reservations and emphasizes that equivalent signals have not yet been observed in other regions where dark matter would also be expected, such as certain dwarf galaxies that are satellites of the Milky Way. It is an absence that will have to be explained.
Anthony Brown, from the University of Durham, in the United Kingdom, recalls that to confirm a signal of this type it is necessary to cross-reference data from multiple forms of radiation, such as radio waves and neutrinos, to exclude other possible causes.
Totani reinforces that the interpretation is still provisional. He says that when he saw the possible signal for the first time, he reacted with skepticism and only gained confidence after thoroughly reviewing the calculations. The hypothesis is exciting, but scientists emphasize that it is still too early to draw firm conclusions.
Just last month, ZAP and other researchers reported what could be a convincing clue in the ongoing hunt to prove the existence of dark matter, in a study published in Physical Review Letters.
