A $2-billion particle detector mounted on the International Space Station has registered an excess of antimatter particles in space, the experiment’s lead scientist announced April 3. That excess could come from fast-spinning stellar remnants known as pulsars and other exotic, but visible sources within the Milky Way galaxy. Or the antiparticles might have originated from the long-sought dark matter, the hypothetical massive particles that constitute some 27 percent of the universe.

Dark matter makes its presence felt by its gravitational pull, but exactly what it is has remained a puzzle. Some popular explanations for dark matter’s identity suggest that when two dark-matter particles collide, they annihilate to produce antimatter electrons, or positrons. The Alpha Magnetic Spectrometer (AMS), delivered to the space station in 2011 during the penultimate space shuttle mission, was built to detect positrons and other high-energy particles streaming through space, in part to investigate the nature of dark matter. The detector has now collected some 25 billion cosmic-ray particles, including 6.8 million electrons and positrons. The fraction of positrons in the particle mix exceeds what would be naively expected in the absence of dark matter or other unaccounted sources, but the new data lack a distinctive feature predicted of dark matter annihilations.

Dark matter collisions would produce relatively more high- than moderate-energy positrons. But the rise in positrons with increasing energy would continue only up to a point. Beyond a certain energy level, the number of positrons would fall off steeply, AMS spokesperson and Nobel laureate Samuel Ting of the Massachusetts Institute of Technology explained in a seminar at CERN, the European laboratory for particle physics. “The positrons could also come from nearby pulsars, and in such a case the positrons will have a slow drop-off” at higher energies, Ting said. “So the way they drop off tells you everything.”

The AMS data indeed show an increasing share of positrons toward higher energies, but no drop-off, so the origin of the excess particles remains unclear. The European PAMELA mission and NASA’s Fermi spacecraft have found similar trends in recent years, but Ting called AMS the first experiment “to probe in detail the nature of this excess with high sensitivity and precision.” The research will appear in the April 5 issue of Physical Review Letters.

Ting only presented data on positrons with energies of about 350 giga-electron-volts or less but said that AMS will in the coming years catalogue particles up to 1,000 giga-electron volts. So the experiment may soon reveal or disprove the presence of a positron cutoff at higher energies, which would provide a clue to the source of the particles: a steep drop would point to dark matter, and a gradual decline would indicate pulsars are the originators of the positrons.

When pressed by colleagues at the CERN seminar to discuss any data AMS has already collected on higher-energy particles, Ting demurred. “We will publish things when we are absolutely sure,” he said, repeatedly sounding notes of caution and calling for patience. “I think that no one is foolish enough to repeat what we are doing,” he said of the experiment, which was some 18 years in the making. “So we want to make sure we are doing it correctly.”