“The gamma rays that we measure are a tracer for the electrons and positrons near the source. Using this, we can map out how fast the electrons and positrons are moving away from the source. Knowing the age and the distance of the pulsars, we can figure out if they can get here,” says HAWC principal investigator and U.S. spokesperson Jordan Goodman of the University of Maryland.Putting this all together in a paper published in the November 17, 2017 issue of Science , the team concludes that the pulsars aren’t producing anywhere near enough positrons to explain the excess observed by PAMELA and AMS. Because the two pulsars are among the closest to Earth, it seems very clear that pulsars in general cannot account for the anomaly.So if pulsars can’t explain the positron excess, what can? Some theorists have proposed supernova remnants and black hole jets. HAWC has detected these types of objects, but as Goodman explains, “Most are too far away and too young to send particles all the way to Earth.”This leaves dark matter particle annihilation as the most likely explanation for the positron excess. This theory has been on the books for many years, and it’s not contradicted by any astronomical observations. Physicists have proposed a number of different types of dark matter particles, with a wide range of properties and masses. If annihilating dark matter is indeed responsible for the positron excess, the particles themselves would have whopping masses of about a thousand protons — approximately the mass of four or five uranium atoms.Experiments at the Large Hadron Collider in Switzerland and in underground laboratories around the world have yet to turn up direct evidence for dark matter particles. So although HAWC seems to have ruled out pulsars as the source of the excess positrons, their origin remains a mystery, as does the nature of dark matter.