Astronomers analyzing two decades of gamma-ray data have detected what appear to be antimatter signals originating from outside the Milky Way for the first time. The discovery suggests our galaxy may be producing significantly more positrons than previously estimated, deepening a long-standing astrophysical mystery.
The findings, published in Astronomy & Astrophysics, stem from the complete dataset of the European Space Agency’s INTEGRAL space telescope. Physicists Thomas Siegert of Julius Maximilian University of Würzburg and Hiroki Yoneda of Kyoto University identified the characteristic gamma-ray flash of matter-antimatter annihilation in regions far beyond our galaxy’s disk.
The Puzzle of Milky Way Positrons
Positrons are the antimatter counterparts of electrons, carrying the same mass and spin but the opposite electric charge. When a positron collides with an electron, the two particles mutually annihilate in a burst of gamma rays that has a highly specific energy signature of 511 kiloelectronvolts (keV). For decades, scientists have mapped this 511 keV emission across the Milky Way to track where positrons are born and where they die.
However, this mapping created a perplexing puzzle. Known astrophysical sources—such as radioactive isotopes forged in supernovae, black holes, and neutron stars—cannot account for the sheer volume of positrons annihilating in the galactic center. The detection of extragalactic antimatter signals only complicates this existing deficit.
A 20-Year Gamma-Ray Survey
The INTEGRAL telescope operated from 2002 until the mission concluded in 2025. Over its 20-year lifespan, it continuously observed the gamma-ray sky, providing an unprecedented archive of high-energy astrophysical data. Siegert and Yoneda were analyzing the complete INTEGRAL dataset to compile the most detailed maps of the Milky Way’s 511 keV emission to date when they noticed anomalous signals outside the expected boundaries of the galaxy.
To rule out instrumental errors or background noise, the researchers rigorously tested the data. They split the 20-year dataset into three-year intervals, comparing the resulting images to each other. They then constructed cumulative datasets, incorporating three years, then six, then nine, and so on. The signals persisted and grew more statistically significant with the inclusion of more data. As Siegert noted, this is not the behavior one would expect from an instrumental background systematic.
Tracing Extragalactic Antimatter
The most compelling signal originated from Complex C, a vast, high-velocity cloud of hydrogen gas currently falling toward the Milky Way disk. A second, more tentative signal was detected from the Magellanic Stream, a massive ribbon of gas trailing behind the Large and Small Magellanic Clouds as they orbit our galaxy. Neither of these regions is expected to produce positrons naturally, which suggests the particles must have traveled there from the Milky Way before annihilating.
This presents a significant physics problem. For positrons to annihilate efficiently, they must be moving very slowly. The popular conception of antimatter violently destroying matter on contact does not apply in the vacuum of space. If positrons need to be slow to annihilate, they theoretically should not possess the energy to escape the Milky Way’s gravitational and magnetic confines. Siegert explained that a fraction of higher-energy positrons must be escaping the galaxy, traveling through intergalactic space, and slowing down in the gas of the Magellanic Stream. This unexpected escape mechanism implies a much larger, more energetic population of positrons than astrophysicists previously modeled.
Rethinking Galactic Antimatter Production
If positrons are indeed escaping the galaxy before annihilation, the Milky Way’s actual positron production must be drastically higher than current estimates. Astronomers typically calculate how many positrons the galaxy produces by observing where they annihilate. If a fraction of those positrons is escaping the galactic disk, the baseline production rate must be adjusted upward.
Siegert and Yoneda estimate that the Milky Way might be producing 100 tredecillion positrons per second. This figure is two to three times higher than previous estimates. “The propagation is indeed an interesting issue here, but I would say that the origin of positrons that are seen to annihilate is then even weirder,” Siegert said. “We have the problem of explaining the sheer number of positrons in the Milky Way. If it turns out to be even more, we are running out of conventional astrophysical sources to explain them.”
The Dark Matter Connection
The implications of this surplus push conventional astrophysical explanations to their breaking point. If standard sources like supernovae and black holes cannot account for the volume of antimatter, scientists may need to turn to unconventional theories. Siegert suggested that at some point, researchers would need to introduce unconventional dark matter models to account for this number of positrons, such as light dark matter particles.
In these models, dark matter particles annihilate or decay into standard matter particles, including positrons, providing a potential source for the observed 511 keV emission. If the excess antimatter signals are confirmed, it could provide a rare, indirect observational pathway to study dark matter, a substance that makes up roughly 27% of the universe but has never been directly detected.
What Happens Next
The current detections remain tentative and carry a reasonable degree of statistical uncertainty. The strongest signal reached 4 sigma, meaning there is roughly a one-in-15,000 chance it emerged from a statistical fluctuation rather than a true astrophysical event. In physics, a 5-sigma certainty—representing a one-in-3.5-million chance of being a fluke—is required to officially claim a discovery.
Siegert is confident that a real annihilation signal exists, but acknowledges that future observations are necessary to determine whether the strength of the signal has been measured correctly. Astronomers will not have to wait long for a new tool to verify these findings. NASA’s Compton Spectrometer and Imager (COSI), scheduled for launch in 2027, is specifically designed to study gamma-ray emissions. COSI should be able to determine whether these faint annihilation signals—which Siegert and Yoneda call “tantalizing hints”—are genuine. If confirmed, the mission could help trace exactly where the Milky Way’s positrons are born, potentially unlocking the source of this cosmic antimatter surplus and fundamentally reshaping our understanding of both galactic physics and dark matter.
— Priya Nair, science desk, AXO News