Positron-annihilation hotspots outside the Milky Way suggest our galaxy makes far more antimatter than thought

Twenty years of data from ESA’s INTEGRAL/SPI spectrometer have produced a new map of the sky at the 511 keV gamma-ray line — the unmistakable fingerprint of electron–positron annihilation — and revealed something unexpected. Several bright hotspots sit outside the plane of the Milky Way, coinciding with the Magellanic Stream, the giant intergalactic gas complex known as Complex C, and the regions around nearby galaxy groups such as Andromeda, Ursa Major, Hydra, Sextans and Virgo.

The team, led by Thomas Siegert, published the analysis in Astronomy & Astrophysics after masking the well-characterized bright emission from the galactic bulge and disk. The high-latitude hotspots that remain had already shown up in a 2025 reconstruction of the same dataset, and now survive further checks.

Positrons escaping the galaxy?

The alignment of the two brightest hotspots with the Magellanic Stream and Complex C is what makes the result striking. It suggests positrons produced inside the Milky Way may be flowing outward and annihilating in surrounding gas — a scenario previous analyses had effectively ruled out, on the assumption that positrons slow down and annihilate long before reaching the outer galaxy.

If real, the additional signal implies the Milky Way is producing positrons at 2 to 3 times the rate estimated so far. Four fainter hotspots pointing toward nearby galaxy groups further raise the possibility that extragalactic annihilation is contributing to the cosmic gamma-ray background — perhaps ~1% overall, but potentially more than 10% above a few MeV under some assumptions.

Artifact or real?

The signal is faint and INTEGRAL’s sky coverage is uneven, so image-reconstruction artifacts remain a genuine possibility. The team argues, however, that all hotspots simultaneously being background fluctuations would be an implausibly consistent coincidence.

NASA’s COSI mission, scheduled for launch in 2027, will map the MeV sky with much greater sensitivity and more uniform coverage. It should be able to confirm — or reject — these extragalactic 511 keV sources.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved.

Publication: Thomas Siegert et al., Possible extragalactic positron annihilation signal, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202659502