We live in a universe dominated by normal matter. This wasn’t always true — right after the Big Bang, in fact, nearly equal amounts of matter and antimatter were created, and then soon afterwards destroyed as they annihilated each other. But because the amounts of matter and antimatter weren’t exactly equal, this annihilation was unequal, too, and normal matter won out.
There is, however, still a small amount of antimatter in our universe, and there seems to be an unexplained excess of it. The reason for this antimatter surplus has long been sought, and now it seems astronomers may have finally arrived at a conclusion: It’s not dark matter responsible for the excess, but plain old pulsars.
The antimatter surplus refers to the fact that a greater number of high-energy anti-electrons, called positrons, than are expected have been detected in space. These detections have been confirmed by several observatories over the past decade, including the Alpha Magnetic Spectrometer on the International Space Station.
Based on our current astrophysical models, the ratio of high-energy positrons to electrons should be tipped significantly in the electrons’ favor. But observations show that there’s an unexpected increase in the ratio of positrons to electrons at energies between about 10 and a few hundred giga-electron volts (GeV).
Astronomers have developed two possible explanations for this excess. One explanation says that dark matter particles (such as weakly interacting massive particles, or WIMPs) randomly annihilating each other could produce positron-electron pairs. Because dark matter accounts for up to 85% of the matter in the entire universe, such interactions could lead to the observed positron excess.
The other explanation isn’t as exotic: The excess could be produced by pulsars and their extremely powerful magnetic fields. These magnetic fields accelerate particles around the pulsar to such high energies that they can generate electron-positron pairs, again bumping up the number of positrons counted by observatories.