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Antimatter may seem like the stuff of science fiction—especially because scarcely any of it can be seen in our universe, despite physicists’ best theories suggesting antimatter should have arisen in equal proportion to normal matter during the big bang. But researchers do regularly produce particles of antimatter in their experiments, and they have the inklings of an explanation for its cosmic absence: Whenever antimatter and normal matter meet, they mutually annihilate in a burst of energy. The slimmest overabundance of normal matter at the beginning of time would have therefore effectively wiped antimatter off the celestial map, save for its occasional production in cosmic-ray strikes, human-made particle accelerators and perhaps certain theorized interactions between particles of dark matter.

That is why physicists were so greatly puzzled back in 2018, when the head of the Alpha Magnetic Spectrometer (AMS) experiment mounted on the exterior of the International Space Station announced that the instrument might have detected two antihelium nuclei—in addition to six that were possibly detected earlier. Any way you slice it, known natural processes would struggle to produce enough antihelium for any of it to end up in our space-based detectors. But the easiest of all those hard methods would be to cook up the antihelium inside antistars—which, of course, do not seem to exist. Despite the fact that the entirely unexpected AMS results have yet to be confirmed, let alone formally published, scientists have taken them seriously, and some have scrambled to find explanations.

Inspired by the tentative AMS findings, a group of researchers recently published a study calculating the maximum number of antimatter stars that could be lurking in our universe, based on a count of currently unexplained gamma-ray sources found by the Fermi Large Area Telescope (LAT). Simon Dupourqué, the study’s lead author and an astrophysics graduate student at the Research Institute in Astrophysics and Planetology at the University of Toulouse III–Paul Sabatier in France and the French National Center for Scientific Research (CNRS), made the estimate after looking for antistar candidates in a decade’s worth of the LAT’s data.

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