For nearly a century, dark matter has been a kind of “cosmic ghost”: we know it’s there because of the gravity it exerts, but no experiment has yet managed to capture a direct trace of its particles . That scenario may be starting to change thanks to a new analysis of data from NASA’s Fermi Gamma-ray Space Telescope.
The work, led by Professor Tomonori Totani of the University of Tokyo, argues that certain gamma rays detected around the center of the Milky Way exhibit an energy and spatial distribution consistent with predictions for the annihilation of theoretical dark matter particles . The study, published in the Journal of Cosmology and Astroparticle Physics , has sparked considerable interest in the international scientific community, including the European one, albeit with a degree of caution.
From Zwicky's suspicion to the attempt to "see" the invisible
The idea that there is an invisible substance that supports the architecture of the cosmos dates back to the 1930s, when the Swiss astronomer Fritz Zwicky observed that some galaxy clusters were moving too fast for the visible mass they contained; it was as if there was a hidden gravitational "scaffolding" missing that would provide additional weight.
Over the decades, cosmological estimates have painted a rather striking picture: only about 5% of the universe is made up of ordinary matter —the matter of planets, stars, and gas. Approximately 27% is attributed to dark matter, and the remaining 68% is believed to be dark energy , another elusive component that accelerates the expansion of the cosmos and, like dark matter, is not directly visible in the light.
Until now, everything known about dark matter came from indirect effects : how it holds galaxies together, how it shapes the formation of clusters and cosmic filaments, or how it perturbs the orbits of stars. But the great leap was missing: detecting a signal associated with the particles that make it up, something that could be described, albeit cautiously, as a possible direct indication.
The problem is that the particles that are candidates for making up dark matter, according to models, do not interact with the electromagnetic force : they do not emit, absorb, or reflect light. This means they are invisible to optical telescopes and most conventional detectors, making their search one of the most intractable challenges in modern physics, including for the numerous European groups dedicated to high-energy astrophysics.
The WIMP hypothesis and the gamma-ray fingerprint
Among the many proposed theories, one of the most influential argues that dark matter is composed of weakly interacting massive particles , known as WIMPs. These particles would be much heavier than a proton, but would barely interact with ordinary matter, except through gravity and the weak interaction.
Theoretical models suggest that when two WIMPs meet, they can annihilate each other and transform their mass into other particles, including gamma-ray photons with very specific energies. This energy signature would act as a kind of "fingerprint" : if an excess of gamma rays is detected with the predicted energy and a spatial distribution consistent with that of dark matter, it could be its trace.
For this reason, regions of the universe where a large accumulation of dark matter is expected have been closely monitored for years , especially the center of the Milky Way and the dwarf galaxies orbiting around it. Observatories around the world, including large gamma-ray telescopes installed in Europe and underground detectors, have joined this patient search.
Despite the technological advancements, results to date had been elusive: fluctuations and small excesses had been recorded in various energy ranges, but none achieved the statistical rigor or consistency necessary to be proclaimed as evidence of WIMP annihilation. The scientific community largely operated between indications and dismissals.
The 20 GeV signal pointing towards the center of the Milky Way
The turning point came when Totani delved into the latest data from the Fermi Gamma-ray Space Telescope , operated by NASA and supported by institutions from different continents. This instrument is designed to record the most energetic photons in the sky, precisely the type of radiation in which the signature of dark matter would be sought.
Following his analysis, the researcher claims to have identified gamma rays with an energy close to 20 gigaelectronvolts (GeV) distributed in a halo shape around the center of the Milky Way. This is not an isolated point source, but rather an extended signal that, according to the study, reproduces the expected shape of a dark matter halo concentrated at the heart of our galaxy.
Furthermore, the energy spectrum of these photons matches what models predict for the annihilation of WIMPs with a mass approximately 500 times that of a proton. Even the estimated frequency of these annihilations remains within the range compatible with theoretical calculations, which reinforces the coherence of the proposed scenario.
In the article published in the Journal of Cosmology and Astroparticle Physics , the Japanese team argues that the signal does not fit well with the usual astrophysical processes operating in the galactic center, such as pulsar activity, supernova remnants, high-energy jets, or diffuse radiation generated by cosmic rays. According to their interpretation, these known sources do not naturally reproduce either the energy or the distribution observed.
The research summarizes the key parameters of the observation in a kind of "technical sheet": photonic energy around 20 GeV , spatial distribution in a halo around the core of the Milky Way, theoretical association with the annihilation of WIMPs and estimated mass of these particles of about 500 proton masses, all supported by the statistical analysis presented in the publication.
The first direct trace of dark matter?
Based on this data, Totani has arrived at a statement that is as suggestive as it is delicate: if his interpretation is correct, this would be the first time humanity has “seen” dark matter . It wouldn't just be a matter of deducing its existence from gravity, but of recording the radiation produced by the particles that compose it as they annihilate each other.
In his statements, the researcher emphasizes that the signal points to a new particle that is not part of the Standard Model of particle physics, the theoretical framework that describes the behavior of the known subatomic world with extraordinary precision. Integrating dark matter into that framework would require extending the current theory with new elements.
For cosmology, such a result would be a key piece: it would allow for better refinement of maps of matter distribution in the universe, a more detailed understanding of how galaxies assemble and why cosmic clusters and filaments have the shapes we observe today with telescopes in Europe and around the world.
In practical terms, having a signal with this energy and morphology would give observing teams a very specific target to track : to look for the same gamma-ray pattern in other dark matter-rich regions, from nearby dwarf galaxies to more distant galaxy clusters, with the collaboration of international telescope networks.
Totani himself, however, insists that the result should still be treated as an indication and not as definitive proof . The signal fits with the predictions, but the center of the Milky Way is an extremely complex region, and there are still possible astrophysical explanations that must be examined closely before declaring victory.
Reactions from the scientific community and the role of Europe
As is often the case with findings that could change established reference manuals, the response from other specialists has been a mixture of interest and caution. Totani himself points out that his results need to be replicated by independent teams and with other analytical methods to gain rigor, something that opens the door to intense observational activity in the coming years.
One of the strategies identified as a priority is to repeat the search for an equivalent signal in dwarf galaxies in the Milky Way's halo. These small satellite galaxies are ideal candidates because they are believed to contain large amounts of dark matter and, at the same time, generate less gamma-ray "noise" than the bustling galactic center.
Astrophysicist Justin Read , from the University of Surrey (UK), points out that so far no clear signals have been detected in dwarf galaxies confirming WIMP annihilation. In his view, this lack of evidence clashes with an overly strong interpretation of the new signal and necessitates a more grounded perspective.
Professor Kinwah Wu of University College London emphasizes the high bar that must be met to accept an announcement of this magnitude: in his words, “an extraordinary claim requires extraordinary evidence ,” and current analysis has not yet reached that level of certainty. Even so, he acknowledges that the work encourages further research with greater precision and better tools.
In Europe, consortia involved in large-scale installations such as the future Cherenkov Telescope Array (CTA) , which will have sites in the northern and southern hemispheres, closely monitor these kinds of results. For these projects, a high-level scientific objective becomes another high-level scientific objective, both in the Milky Way environment and in clusters and other large-scale structures.
Next steps and what still needs to be clarified
One of the essential points from now on is to determine whether the 20 GeV signal can be reproduced using known astrophysical sources . The galactic center contains remnants of supernova explosions, pulsar populations, hot gas clouds, and intense background radiation—factors that overlap and complicate any attempt to isolate a specific contribution.
For this reason, many researchers emphasize the importance of expanding the analysis to less polluted environments , such as the dwarf galaxies surrounding the Milky Way or certain clusters where background emission is more manageable. If an excess of gamma rays with the same energy and a distribution consistent with the presence of dark matter is detected in these objects, the argument in favor of WIMPs would gain considerable strength.
Verifying the result will require both new observation campaigns with Fermi and other space- and ground-based telescopes, as well as more refined statistical techniques capable of clearly separating the potential dark matter signal from other emissions. In this effort, collaboration between teams in Japan, Europe, and other countries will be crucial.
Meanwhile, theorists are exploring the implications of the particle responsible for dark matter actually being a type of WIMP with a mass on the order of 500 times that of the proton . If confirmed, particle physics would have to extend the Standard Model to accommodate this new entity and revise many current hypotheses about physics beyond that framework.
In parallel, cosmology would take advantage of this possible advance to more precisely adjust the distribution of matter in the universe and test models on the formation and evolution of galaxies, something in which multiple European observatories and international space missions collaborate.
For now, everything points to dark matter still playing hide-and-seek, but the idea that we could be facing its first direct trace marks a change of stage: the exploration of the universe no longer relies solely on visible light or its best-known variants, but also on subtle energetic traces that could reveal what, until now, has remained completely hidden from our eyes and our instruments.
