Mysterious Origins of Antimatter Continue to Baffle Scientists as Gamma Ray Observations Point to Dark Matter

A giant array of water tanks, HAWC, lets scientists study gamma rays. HAWC

Scientists have long been wrestling with a mystery: Why is Earth being pummeled by a stream of teeny, tiny positively charged particles known as positrons? One possible source is the much-hunted dark matter, but there are other possible explanations, too, like that the stuff is coming from bright, star-like objects called pulsars.

But the pulsar explanation just became a lot less plausible, according to a new paper published today in the journal Science. That conclusion is based on some of the earliest data gathered by a device known as the High-Altitude Water Cherenkov Observatory, or HAWC, which is a set of giant vats of water located in Mexican mountains that picks up signals generated by positrons and lets scientists see where they came from.

"It's worked even better than we had hoped it would work," co-author Jordan Goodman, a physicist at the University of Maryland, told Newsweek. "HAWC observes the entire sky overhead—all the time, 24 hours a day." As it does, the water vats it's made up of act sort of like film negative to gather gamma rays, which are a super-high-energy form of light.

"The thing that makes these objects easy for us to see—and also difficult for everyone else to see—is their extent," Goodman said. HAWC picks up what's called Cherenkov radiation, which is light produced by particles that are moving faster than the speed of light.

Other detectors that gather Cherenkov radiation are picky, with relatively small fields of view. "You kinda gotta know what to look at, and they're not good at seeing big objects," Goodman said, comparing it to how human eyes are attuned to recognize boundaries. "If you just look at a gray slab, you won't know it's an elephant unless you look at the edges."

Not so HAWC, which delivers a picture of the whole sky. That let the team behind the new paper trace all of the radiation coming out of two nearby pulsars that had been considered good candidates to be the source of all these extra positrons in our celestial neighborhood. They found that the results don't quite match up with what they would have expected, given how we think pulsars work.

Read more: Search for dark matter dealt another blow as another hiding place for elusive substance ruled out

But HAWC isn't done yet—the detector has already gathered almost twice as many days' worth of data as were included in the current paper. That data will let scientists create a more detailed picture against which to vet their new theories about where all those positrons are coming from. Those theories could include more complicated ideas about pulsars or other sources entirely.

For now, that source is still a mystery, and the team isn't eager to point any fingers. "We haven't discovered dark matter," Goodman said. "We're just adding evidence to the pile, and in this case it happens to be on the side of dark matter."