Seeking the unseen (Image: NASA)

A dark matter-hunting telescope perched on the International Space Station has spotted millions of particles of antimatter. It could be the first clear evidence of dark matter particles smashing into each other – or something much more mundane.

“It’s an indication, but by no means is it a proof” of dark matter, says Nobel laureate Samuel Ting of the Massachusetts Institute of Technology, the principal investigator for the Alpha Magnetic Spectrometer experiment.

AMS launched on the final flight of the space shuttle Endeavour in May 2011 in order to catch whiffs of the most exotic types of matter. That includes dark matter, thought to make up about 80 per cent of the matter in the universe, but which scarcely interacts with ordinary matter and so has never been conclusively detected.


Today, at a seminar at the CERN particle physics laboratory near Geneva, Switzerland, Ting reported that AMS has seen more than 30 billion cosmic rays, charged particles of mysterious origin that constantly stream through space. The telescope’s magnetic detector identified 6.8 million of them as electrons or their antimatter counterpart, positrons.

If dark matter particles meet in space and annihilate each other, they should decay into electrons and positrons in equal number, raising the overall level of positrons relative to electrons.

Same signal across space

Ting’s team found that the ratio of positrons to electrons goes up at energies between 10 and 350 gigaelectronvolts, although the rise is not sharp enough to conclusively attribute it to dark matter collisions. They also found that the signal looks the same across all space. That is what you would expect if the signal is due to dark matter, as the mysterious stuff is thought to cluster evenly across the galaxy.

Previous telescopes, like the Fermi and PAMELA gamma-ray instruments, saw a similar rise, although not with AMS’s precision and not up to the same high energies. “By itself it’s not surprising,” Ting says. “Many people have seen it, but it was always seen with very large systematic errors. This is the first time we have been able to, in detail, like with a microscope, figure out what’s going on.”

Ting also sees a hint of interesting new physics near the edge of the data the team has analysed so far. The slope of the positron ratio seems to flatten out at about 350 gigaelectronvolts. If it dives sharply at higher energies, that would be a sign of dark matter, Ting says, because there should be a window of energies that corresponds only to dark matter particles. If not, then the positrons could come from more mundane sources, like spinning stars called pulsars. “Our main point is that we published this data, and now let the community work on it,” Ting says.

Gregory Tarle of the University of Michigan, who worked on previous cosmic ray telescopes but is not involved in AMS, is pleased that AMS confirmed that there are more positrons than expected at higher energies – but is sceptical that they can be definitively attributed to dark matter.

“There are so many knobs you can turn in these models,” he says. “It would surprise me no end if a physicist couldn’t account for anything that they saw using an astrophysical source”, such as a pulsar instead of dark matter.

The AMS results have been hotly anticipated. Ting was expected to announce them in February but changed his mind at the last minute, preferring to wait until they were published in a journal. They have been accepted for publication by Physical Review Letters.

Ting is scheduled to announce another update to the AMS results at the International Cosmic Ray Conference in Rio De Janeiro, Brazil, in July.