International Space Station
The International Space Station has detected high levels of positrons in cosmic rays, suggesting they come from mysterious dark matter.NASA

An unexpected excess of positrons in cosmic rays could reveal the nature of dark matter.

By examining 30 months of data from the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station (ISS), MIT's Laboratory for Nuclear Science has discovered the positron flux significantly differed from the electron flux above 30 GeV in energy, suggesting that positrons and electrons have different origins.

Cosmic rays are predominantly made of protons and other particles, but electrons and positrons make up part of the mix.

Standard astrophysical models of interstellar particle collisions predict that in cosmic rays the amount of positrons relative to electrons should decrease with energy.

However, observations from satellites and the AMS have shown that the amount of positrons increases with energy.

The AMS is a large particle detector on the ISS that captures incoming cosmic rays from all over the galaxy, and discriminates between different particles.

Among 41 billion cosmic ray particles, the researchers identified 9.2 million electrons and 580,000 positrons –particles with the same mass as electrons, but the opposite electrical charge.

Extending measurements of the positron flux up to 500 GeV and, similarly, of the electron flux up to 700 GeV they saw that both spectra change their slope at around 30 GeV but behave differently towards higher energies: the positron flux drops off much more slowly than the electron flux.


An excess of positrons at high energies is being interpreted to suggest they may be originating from a new source. Possibly collisions between dark matter give rise to positrons. Data at higher energy will be needed to confirm a dark matter origin.

"The new AMS results show unambiguously that a new source of positrons is active in the galaxy," says Paolo Zuccon, an assistant professor of physics at MIT.

"We do not know yet if these positrons are coming from dark matter collisions, or from astrophysical sources such as pulsars. But measurements are underway by AMS that may discriminate between the two hypotheses."

The new measurements are compatible with a dark matter particle with mass in the order of 1 teraelectronvolt (TeV) — about 1,000 times the mass of a proton.

According to theoretical predictions, when two dark matter particles collide, they annihilate, releasing energy and ordinary particles that eventually decay into stable particles, including electrons, protons, antiprotons, and positrons.

While the visible matter in the universe consists of protons and electrons, not much of this results from dark matter collisions.

However, positrons and antiprotons are much rarer in the universe and their detection above a very small expected range must come from a new source.

Dark matter is believed to make up over 85% of the matter in the universe. Invisible to radiation, they are only perceived from invisible effects like gravity they must exert to keep galaxies from spinning away.

Zuccon and his colleagues, including AMS's principal investigator, Samuel Ting, the Thomas D Cabot Professor of Physics at MIT, detail their results in two papers published in the journal Physical Review Letters.