The way stars and galaxies move shows there is more mass present than we can see. To account for this, 80 per cent of the universe's matter must be dark. No wonder physicists are desperate to find it. The trouble is the stuff stubbornly refuses to interact with ordinary matter, except through gravity, so has not been conclusively detected.
The most favoured models say it is made up of a new, weakly interacting massive particle. These WIMPs collide in space, annihilating and decaying into ordinary particles, including electrons and their antimatter counterparts, positrons.
Since May 2011, the Alpha Magnetic Spectrometer has been sitting on the International Space Station, sifting through billions of charged cosmic rays for evidence of those annihilations. If it sees an excessive number of positrons relative to electrons at a certain energy, that might just be a compelling sign of dark matter.
On 3 April, AMS designer Samuel Ting of the Massachusetts Institute of Technology reported an expected rise in the ratio of positrons to electrons at energies between 10 and 350 gigaelectronvolts. Frustratingly though, the upturn is not yet sharp enough to attribute to dark matter collisions since the extra positrons could still come from more mundane sources like pulsars. "It's an indication, but by no means is it a proof," Ting says.
In the meantime, a further quirk in the results suggests that if the particles are dark matter, they may not be vanilla WIMPs.
The simplest models predict there should be only a certain amount of dark matter hanging around, and that WIMPs should rarely meet. But AMS has spotted too many positrons for that – so what could make WIMPs collide in space more often than expected?
In 2008, when the PAMELA satellite found a similar excess of positrons, Neal Weiner of New York University and colleagues suggested that WIMPs are drawn together under a force of their own. This new force increases their collision rate but would have escaped our gaze until now