Seventy years ago, scientists first calculated that galaxies must contain additional, undetectable sources of mass — up to five times the mass of the detectable gas and stars. Piergiorgio Picozza, a physicist at the University of Rome Tor Vergata in Italy, has spent his career searching for this invisible 'dark matter', which is proposed as the source of the added mass, and he might now have found evidence for it.

Picozza has been investigating the formation of antimatter in space. Antimatter consists of particles that have the same mass as electrons and protons, but opposite properties such as charge. For example, the positively charged positron is the antimatter counterpart of the electron. Positrons can be produced by 'secondary processes', such as cosmic-ray nuclei smashing into interstellar dust, which occur at relatively low energies, but they might also arise directly from 'primary sources', such as dark-matter annihilations, that could generate positron–electron pairs at high energies. The latter process has not yet been confirmed. So a better understanding of positron formation could indicate the presence of dark matter. “A very important part of our job is to disentangle the sources of positrons,” says Picozza.

To gather the necessary data, Picozza organized a collaboration of Russian, Italian, German and Swedish colleagues dubbed PAMELA — Payload for Antimatter–Matter Exploration and Light-nuclei Astrophysics. At first, PAMELA was difficult to get funded as a US-led collaboration had just begun similar work. But Picozza persevered and convinced European funders that two sets of data would be better than one. Specialized high-energy particle detectors to precisely measure the abundance of cosmic rays, electrons, positrons and other antimatter particles were sent into Earth orbit on board a satellite in 2006.

To identify possible primary source antimatter production, the team focused its analysis on the energy interval between 1.5 and 100 gigaelectron volts (GeV). If positrons are produced mainly from secondary sources, the ratio of positrons to electrons detected would be expected to decrease with increasing energy. But, surprisingly, the team found that this fraction increased significantly between 10 GeV and 100 GeV (page 607). The authors conclude that a primary source is needed to generate the high numbers detected at these higher energies.

Picozza is careful not to jump to the conclusion that their results prove that the primary source of antimatter is dark-matter annihilation. Pulsars, relics of massive stars that emit radiation, could also generate positrons. The ultimate confirmation that antimatter particles are produced from dark matter will come only if the Large Hadron Collider (LHC) at CERN near Geneva in Switzerland can experimentally produce 'dark matter particles'. “I remain open-minded about the possibilities, but if the LHC confirms our data, it would easily be the best result I — and more importantly, my young collaborators — will have achieved,” says Picozza.

Until then, he hopes to take advantage of PAMELA's remaining time in space to follow antimatter production during a shift from low to high solar activity. The PAMELA data below 10 GeV were obtained in a period of low solar activity, and are remarkably different from previous data obtained during high activity.