Kate Becker The Visible Universe
Kate Becker The Visible Universe

The smell started on a Thursday, sweetish and earthy in a there-goes-the-garbage truck sort of way. Herbal tea? Spoiled air freshener? Everyone had a guess.

By Friday, things were getting worse. Garbage disposal? Mildewed laundry? Obviously, something was causing it. But even though the apartment was searched top to bottom, from under the couch to over the cupboards, the source was a mystery — kind of like dark matter.

For decades, astronomers have been tracking the scent of dark matter all across the cosmos. There it is, its gravity grasping stars along the periphery of distant galaxies, preventing them from flying off into intergalactic space. There it is again, imprinted on the echoes of the Big Bang — and over there, bending light from faraway galaxies. Add it all up, and dark matter seems to outweigh normal matter five-to-one. But what exactly is it?

Physicists think that dark matter is probably made up of some subatomic particle they haven't found yet, something relatively heavy that interacts with other particles only when it's nose-to-nose with them. But despite years of searching, physicists haven't detected any dark matter particles — at least not any they're sure of.

While physicists try to catch a glimpse of dark matter with detectors here on Earth, astrophysicists are hoping dark matter might be able to resolve another cosmic mystery: the origin of a glut of antimatter particles called positrons. In 2008, a satellite called PAMELA found that Earth is surrounded by a surprising abundance of these particles. The measurement that has been confirmed by two more experiments since. But where are all the positrons coming from?


The plain-vanilla explanation (actually pretty exotic in its own right) is that the positrons originate in dead stars called pulsars. These stars can spin more than a thousand times per minute, shedding a "wind" of electrons and positrons which gets a second kick of speed when it hits a surrounding shock wave. This wind also bumps up the energy of nearby photons, creating a gamma-ray glow.

But there's another potential explanation for the positrons. If dark matter particles annihilate each other when they meet, their collisions could shower the universe with positrons. This hypothesis is more speculative, balanced on a stack of "ifs" — if dark matter particles annihilate, if they behave the way we think they do — so the only way to "rule it in" is to rule all other explanations out.

Now, researchers may have come a step closer to ruling out the pulsar hypothesis. To test it, a team of researchers used a telescope called the High-Altitude Water Cherenkov (HAWC) Observatory, a grid of 300 water tanks positioned at a 2 1/2-mile elevation in the Mexican state of Puebla, to measure gamma rays coming from two nearby pulsars. The pulsars are just the sort posited to be the source of the extra positrons. But the results suggest that the pulsar particles aren't moving fast enough to explain the positron surplus.

Not everyone is convinced that this is the right reading of the gamma-ray measurements, and even if it is, it does not mean that the dark matter hypothesis is right — only that researchers must look beyond pulsars for an answer.

Meanwhile, the hunt for dark matter goes on, with researchers slowly peeling back layer after layer of mystery, like an onion — which, incidentally, was discovered to be the source of the unlucky smell. That's one mystery down, and many, many more to go.

Kate Becker is a science writer living in Boston. Contact her at spacecrafty.com, or connect via facebook.com/katembecker or twitter.com/kmbecker.