Is Antimatter Escaping the Milky Way? Read the Sigma First

Somewhere in the Milky Way, something is making a huge amount of antimatter, and for decades nobody has been able to say what. The signature is a glow of gamma rays at exactly 511 kiloelectronvolts, the energy released when a positron, the antimatter twin of the electron, meets an electron and both vanish. A paper published in Astronomy & Astrophysics in July 2026, and widely covered this week, raises the possibility that the puzzle is even bigger than thought: some of those positrons may be escaping the galaxy and annihilating in gas clouds outside it.

The positron puzzle

The 511 keV line has been observed for decades, and it is brightest toward the center of the galaxy. The trouble is quantity. Astronomers estimate how many positrons the Milky Way produces by counting where they annihilate, and the count is uncomfortably high for the known sources, such as black holes, neutron stars and other high energy objects. The authors of the 2025 map call it, simply, the positron puzzle, one of the long running mysteries of high energy astrophysics.

That map is the foundation of the new claim. Hiroki Yoneda, Thomas Siegert and Saurabh Mittal analyzed 20 years of observations from the SPI spectrometer aboard the European Space Agency's INTEGRAL gamma ray observatory, covering February 2003 to August 2023, and built the most sensitive all sky image of the 511 keV line to date. It was published in Astronomy & Astrophysics in October 2025. Alongside the bright central bulge and the galactic disk, the image showed several faint hotspots far from the plane of the galaxy.

What the new paper proposes

Those hotspots could be imaging artifacts. Or they could be real. In the follow up paper, "On the Possibility of an Extragalactic Positron Annihilation Signal," Siegert, at the University of Würzburg, and Yoneda, at Kyoto University, compared the hotspots with maps of high velocity gas clouds around the Milky Way and with a catalog of nearby galaxies out to about 25 megaparsecs.

Two of the brightest hotspots line up with known gas structures: the Magellanic Stream in the southern sky and a cloud called Complex C in the northern sky. If positrons made inside the Milky Way escape and annihilate in those clouds, the authors write, the galaxy's total positron production would be higher than the rate measured from annihilation in the galactic interstellar medium alone, on the order of 10⁴⁴ positrons per second. They also suggest that some of the other hotspots could be explained by the combined glow of nearby galaxies.

The title of the paper starts with the words "On the possibility." That is the most important finding in it.

How strong is the signal

Not strong. In the 2025 map paper, the estimated significance of individual high latitude regions ranged between 2 and 4 sigma. For comparison, physics usually asks for 5 sigma before calling something a discovery. The authors themselves frame the hotspots as either artifacts or true signals, and the new paper is built on the second possibility without claiming it is proven.

The match with gas clouds is suggestive, because it gives the positrons somewhere physical to die. But a spatial coincidence on a deconvolved gamma ray map, where the image is reconstructed from a detector that does not take pictures in the usual sense, is the kind of result that needs independent confirmation. The same caution applied to the K2-18b life signal: a few sigma is a reason to look again, not a conclusion.

How it gets settled

INTEGRAL itself is done. It stopped collecting science data in February 2025 after more than two decades in orbit, so no amount of patience will make its map sharper. The 2025 paper points to the next generation of MeV gamma ray telescopes, naming NASA's Compton Spectrometer and Imager, COSI, as the kind of instrument expected to confirm or dismiss the reported features. COSI is scheduled to launch in 2027, according to coverage of the study.

The new paper also offers a test that could cut either way. If the interpretation is right, future instruments should be able to pick out individual extragalactic 511 keV sources for the first time, including several dwarf galaxies around the Milky Way and the Andromeda and Triangulum galaxies, M31 and M33. A clear prediction is the best thing a tentative claim can offer, because it can fail.

If it holds, the answer to the positron puzzle gets harder, not easier: the Milky Way would be making even more antimatter than the already unexplained amount astronomers can see. The galaxy may not be hiding a source. It may be hiding the evidence.