Milky Way Survey Finds No Dark Matter Signal, But Narrows the Search

An array of five gamma-ray telescopes trained on the center of the Milky Way for six years has failed to find the signature scientists have sought for decades as proof of dark matter, but the null result is itself the most stringent constraint yet placed on a leading theoretical candidate for the substance that makes up most of the universe's mass.

The H.E.S.S. Collaboration, the international team that operates the High Energy Stereoscopic System in Namibia, reported in a paper published in Physical Review Letters that it searched 546 hours of observations collected between 2014 and 2020 for a narrow "gamma-ray line," a telltale concentration of light at one specific energy that several models predict would appear if two dark matter particles collided and annihilated each other near the galaxy's core. No such line turned up. Instead, the collaboration used the absence of a signal to rule out large swaths of the parameter space available to some of the most closely watched dark matter candidates, including, for the first time under the galactic models the team tested, a particle known as the thermal Higgsino.

What the telescopes were looking for

Dark matter is inferred entirely from its gravitational effects; it has never been directly detected, and its particle identity remains one of physics' oldest open questions. One long-favored hypothesis holds that dark matter consists of weakly interacting massive particles, or WIMPs, which occasionally meet and transform into ordinary particles, including high-energy photons. If two WIMPs annihilate promptly into two photons, the result should be a sharp spike in gamma rays at the particles' mass energy rather than the smoother spread expected from background astrophysical sources, a signature that would be difficult to explain any other way.

The Milky Way's galactic center is the prime hunting ground because theoretical models expect dark matter to pool there in especially high density. H.E.S.S., an array of four 12-meter telescopes plus one larger 28-meter instrument in Namibia's Khomas Highlands, detects these candidate signals indirectly, by catching the faint blue flashes of Cherenkov light that gamma rays produce when they strike molecules high in Earth's atmosphere. "These particles arrive in cascades, or showers, initiated by high-energy photons from the universe," H.E.S.S. researcher Alessandro Montanari explained, describing how the technique lets ground-based telescopes reconstruct the energy and direction of gamma rays that never reach the surface directly.

The numbers behind a non-detection

Working through 61 energy bins spanning 300 GeV to 64 TeV and 25 spatial regions around the galactic center, the team tested five different models of how dark matter might be distributed through the Milky Way's halo, including the widely used Einasto profile. Under that profile, the collaboration pushed the upper limit on the annihilation cross section — essentially a measure of how readily two dark matter particles would merge and disappear — down to about 2.3×10⁻²⁸ cubic centimeters per second for a particle with a mass of roughly 1 TeV, and roughly 2.4×10⁻²⁷ cubic centimeters per second at 10 TeV. Those figures roughly double the strength of H.E.S.S.'s own earlier constraints at the lower end of that mass range and represent, according to the collaboration, the tightest limits so far obtained anywhere on multi-TeV dark matter annihilating into gamma-ray lines.

The practical consequence is that two theoretical dark matter candidates known as the thermal Wino and the thermal Quintuplet are now excluded across every halo profile the team examined, while the thermal Higgsino — a supersymmetric particle whose predicted mass near 1 TeV has made it notoriously difficult to rule out by any method — faces a meaningful experimental challenge for the first time under these galactic-center observations.

Two decades of pointing at the galactic core

H.E.S.S., whose name honors the Austrian physicist Victor Hess, has been observing the sky from Namibia since 2002, with its largest telescope added in 2012. The Max Planck Institute for Nuclear Physics, which helps lead the collaboration together with France's CNRS/IN2P3 and dozens of partner institutions, has used the array's position in the Southern Hemisphere to keep the Milky Way's center in view for long stretches each year, a geometric advantage no other currently operating Cherenkov telescope array shares. "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 gamma rays, the H.E.S.S. observatory acquired enough data to probe the relevant annihilation cross section of WIMPs," said Emmanuel Moulin, a researcher with the collaboration.

That patience is what let the team assemble one of the largest single datasets of its kind. "We obtained the most constraining results on the supposed annihilation cross section of heavy dark matter particles," Montanari said. "This was possible because H.E.S.S. is currently the only active array of [imaging atmospheric Cherenkov telescopes] 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 [such] observations of this region available at the time of writing."

What a null result changes

For particle physicists and cosmologists, a non-detection with this level of precision narrows the map rather than closing the search. Dark matter is estimated to make up roughly 85 percent of all matter in the universe, yet every attempt to detect it directly or produce it in a collider has so far come up empty; indirect searches like H.E.S.S.'s instead squeeze the space of masses and interaction strengths that remain viable for any given theoretical particle.

"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," Montanari said, describing the quantity the collaboration worked to constrain.

That framing matters because the thermal Higgsino has effectively eluded every other experimental approach, including underground detectors and particle colliders, precisely because a particle with its predicted mass and feeble interaction strength is so hard to produce or catch directly. Showing that even the deepest gamma-ray survey of the galactic center to date cannot yet rule it out — while finally beginning to challenge it — tells theorists how much observational power is still needed before the Higgsino hypothesis can be confirmed or discarded.

A sharper instrument is already under construction

The collaboration says the result also sets a benchmark for the instrument meant to succeed it. The Cherenkov Telescope Array Observatory, now being built at two sites — CTAO-South near Paranal, Chile, and CTAO-North at the Roque de los Muchachos Observatory on La Palma in the Canary Islands — is designed with a larger field of view and sharper angular and energy resolution than any array operating today. "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," Moulin said.

Until CTAO-South comes fully online, H.E.S.S. remains the only Cherenkov array with a clear, sustained view of the Milky Way's center, and the collaboration says it will continue adding observing time in hopes of either tightening these limits further or, eventually, finding the line it has spent more than twenty years looking for.