Dark matter could be a type of matter in the universe that does not emit, absorb or reflect detectable light and appears to interact very weakly with regular matter. Although physicists have observed gravitational effects attributed to dark matter, they have not yet been able to determine what it is made of.
One hypothesis is that dark matter consists of particles that sometimes annihilate, meaning that when two meet, they transform into other particles. Some hypothetical dark matter particles are predicted to produce gamma rays (i.e., a highly energetic form of light) during this process.
A hint of these particles’ presence could be an unusual concentration of gamma rays at a particular energy, instead of a smooth spread across different energies. Such a concentration, called a gamma-ray line, might be detectable toward the center of our galaxy, the Milky Way, where dark matter is thought to be especially dense.
The H.E.S.S. (High Energy Stereoscopic System) Collaboration, a large group of researchers at institutions worldwide, recently searched for a gamma-ray line using observations collected by the collaboration’s array of telescopes in Namibia. These telescopes detect flashes of light produced when incoming gamma rays strike Earth’s atmosphere.
The results of this research, published in a paper in Physical Review Letters, did not yield a convincing dark matter signal but set new limits on the possible annihilation of proposed dark matter particles.
“Dark matter in the form of elementary massive electroweak particles (WIMPs) is among the most favored candidates to explain dark matter in the universe, from astrophysical to cosmological scales,” Emmanuel Moulin, a researcher with the H.E.S.S. Collaboration, told Phys.org.
“After looking for WIMPs for more than two decades in the center of the Milky Way, long acknowledged as the most promising target to detect WIMPs via their self-annihilation in very-high-energy (E>100 GeV) gamma rays, the H.E.S.S. observatory acquired enough data to probe the relevant annihilation cross section of WIMPs.”
Seeking a distinctive signal from the galactic center
The H.E.S.S. Collaboration is the international research team that operates the H.E.S.S. system and analyzes its data. The system is an array of imaging atmospheric Cherenkov telescopes (IACTs) located in Namibia, in southern Africa.
“The data we analyzed consist of Cherenkov flashes of light generated by particles traveling faster than the speed of light in Earth’s atmosphere,” explained Alessandro Montanari, a researcher with the H.E.S.S. Collaboration.
“These particles arrive in cascades, or showers, initiated by high-energy photons from the universe. With this technique, we could observe the most energetic phenomena, such as what we believe could happen when two dark matter particles annihilate promptly into two high-energy photons—what we call a spectral line from dark matter annihilation.”
Moulin, Montanari and their colleagues analyzed 546 hours of observations collected by the H.E.S.S. telescopes between 2014 and 2020, covering the region around the center of the Milky Way. The team looked for narrow concentrations of gamma rays at a specific energy, which some dark matter models predict could result from particle annihilation.
“We obtained the most constraining results on the supposed annihilation cross section of heavy dark matter particles,” explained Montanari. “This was possible because H.E.S.S. is currently the only active array of IACTs in the Southern Hemisphere—meaning it is perfectly positioned to observe the center of the Milky Way—and we used one of the largest data sets of IACT observations of this region available at the time of writing.
“Think of the annihilation cross section as the size of the particle when it collides with another one, and they both disappear—or, more specifically, transform into something else, like the two photons we mentioned before.”
Setting stronger limits on dark matter annihilation
The H.E.S.S. Collaboration did not observe a statistically significant gamma-ray line in the analyzed data. Nonetheless, the team could set upper limits on the predicted annihilation signal that could inform future dark matter searches.
In the context of a specific model of how dark matter is distributed in the Milky Way (i.e., the Einasto model), the researchers set the strongest limits yet on the gamma-ray line signal they sought. Their results also challenge the thermal Higgsino, a proposed dark matter candidate, for the first time under the galactic dark matter models they examined.
“Our analysis sets the smallest upper limit for this size across a wide range of possible dark matter particle masses,” explained Montanari. “We test all mass values we can cover within our telescopes’ energy range because we don’t know the dark matter particle’s mass.”
Future searches with more sensitive gamma-ray telescopes could build on these results, setting stronger limits or potentially identifying a gamma-ray signal linked to dark matter annihilation.
“The Cherenkov Telescope Array Observatory (CTAO) is being built at two sites, CTAO-South, located in Paranal, Chile, and CTAO-North, situated at the Roque de los Muchachos Observatory on La Palma, Canary Islands, with improved sensitivity due to a larger field of view and acceptance and higher angular and energy resolutions compared to currently operating imaging atmospheric Cherenkov telescopes such as H.E.S.S.,” added Moulin.
“The observation program carried out with H.E.S.S. is an important legacy and paves the way for future planned observations of the galactic center with CTAO, which will provide crucial insights into the TeV WIMP paradigm as a whole.”
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Publication details
F. Aharonian et al, Search for Gamma-Ray Spectral Lines from Dark Matter Annihilation with the H.E.S.S. Inner Galaxy Survey, Physical Review Letters (2026). DOI: 10.1103/d8tj-55kc. On arXiv: arxiv.org/abs/2608.07234
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Gamma-ray search sets new limits on dark matter annihilation in the inner Milky Way (2026, October 4)
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