This thread is prompted by apod Space Station Detector Finds Unexplained Positron Excess and by the associated technical article

If I was asked what the ratio of antimatter to matter in the universe was, I might guess one in a thousand, or one in a million. But at energies above 100 GeV, three detectors are now telling us that the ratio is close to 1 in 5, one positron for five electrons.

The number of electrons in the universe (multiplied by the cube of the particle energy) peaks at 7 GeV and at higher energies declines to a plateau. The number of positrons on the other hand continues to rise above 7 GeV, also towards a plateau.

“Where did all these high energy positrons come from? The Alpha Magnetic Spectrometer (AMS-02) onboard the International Space Station (ISS) has been meticulously recording how often it is struck by both high energy electrons and positrons since 2011. After accumulating years of data, it has now become clear that there are significantly more positrons than expected at the highest energies detected. The excess may have a very exciting and profound origin — the annihilation of distant but previously undetected dark matter particles. However, it is also possible that astronomical sources such as pulsars are creating the unexplained discrepancy. The topic remains a very active area of research.”

Results from the AMS match those from earlier experiments, notably PAMELA and FERMI, but have a much higher accuracy. A news release about the PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) results from Aug 2013 can be found here

“Theorists have put forward several explanations for the positron excess. The most exciting involves dark matter, an elusive substance that interacts with gravity but not light, and that is estimated to make up nearly 85% of the total amount of matter in the universe. According to many theoretical models, dark-matter particles can annihilate one another, thereby generating electrons and positrons. At low energies, electrons from other astrophysical sources would easily outnumber these positrons, but with increasing energy such electrons would dwindle, leaving the numbers of electrons and positrons to start balancing out, as PAMELA observed.”

“Another explanation is that positrons are being generated in spinning stars known as pulsars. And yet another possibility is that the PAMELA team has misunderstood its experiment. Misinterpretation now seems almost totally unlikely because early last year physicists using NASA’s Fermi Gamma-ray Space Telescope confirmed that they had also detected a rise in the positron fraction at energies of between 20 and 100 GeV. Then in April this year, data from the Alpha Magnetic Spectrometer (AMS) experiment aboard the International Space Station became the third to reveal a positron excess.”