An orbiting observatory may have found the first indirect evidence of dark matter particles colliding in space and disappearing, as if in a puff of smoke.

The “smoke” in this case consists of positrons, the antimatter counterpart of electrons. The constant rain of energetic particles that bombards the Earth’s surface, known as cosmic rays, contains many more positrons than scientists expected, according to a study in Nature Wednesday.

All theories agree that dark matter must give this signal, an increasing of number of positrons,” said Piergiorgio Picozza of the University of Rome, leader of the study.

Positrons and other particles of antimatter can enter the stream of cosmic rays in three ways. One is for cosmic rays to collide with stray atoms in interstellar space, producing a shower of particles. Known as a

“secondary source,” this process is similar to what happens inside particle accelerators, and scientists presumed it was where most positrons came from. Another possibility is that they are produced in the magnetic fields of pulsars, rapidly spinning stellar leftovers from supernova explosions, or microquasars, small, distant galaxies with active cores.

The third and most exciting option is the collision of dark matter particles. The top candidates for dark matter, the heavy but invisible stuff that makes up 23 percent of the universe, are weakly-interacting massive particles. Contrary to their WIMPy name, when two of these particles collide, they annihilate each other in a burst of energy and propel a cloud of matter and antimatter particles into space. The theory has been a favorite of physicists for years, but until now, no one had detected evidence of these collisions.

To measure the abundance of positrons in cosmic rays, the team used data from the instrument PAMELA (Payload for Antimatter Matter

Exploration and Light-nuclei Astrophysics), which launched aboard a Russian satellite in June 2006. Unlike previous antimatter-hunting instruments, PAMELA can pinpoint not just the type of incoming particle but also its energy.

The team calculated the fraction of incoming particles that positrons at several different energies accounted for. They found that as the energy went up, so did the percentage of positrons. This upswing rules out secondary sources as the main source of positrons, and bolsters the case for dark matter.

This isn’t the first time this idea has come up. In August, the PAMELA team cautiously presented these results at two conferences in Stockholm and Philadelphia, sparking a flurry of activity in the physics world. Some enterprising physicists snapped photos of the presentation’s slides and extracted the data to analyze it themselves.

In response, the PAMELA team released their data on the preprint site arXiv.org in October. More than 100 papers have come out since then, and more than half of them argue for dark matter sources.

But not so fast. The same team published a study in February saying that a similar measurement of anti-protons could be explained just from cosmic rays hitting interstellar dust, with no need for dark matter at all. “The data significantly constrain contributions from exotic sources, e.g., dark matter,” the team wrote. The physics community sighed — maybe it's not dark matter after all.

This apparent contradiction doesn't bother Picozza. Both results narrow down the possibilities. “There are many models,”

he said. “We did not see anything for anti-protons, so those models are more or less ruled out, or they have to change something. But there remain many other models that prefer all positrons.”

Pulsars are still an equal contender. Other physicists are cautious about jumping on the dark matter solution. "It’s a very interesting find, but we don’t know yet if we need to invoke some exotic explanations," said physicist Yousaf

Butt of the Harvard-Smithsonian Center for Astrophysics. "There are certainly other prosaic explanations."

More data from

PAMELA at higher energies combined with observations from other observatories will help determine which source produces more positrons.

“Pulsars are less exotic, but still very important,” Picozza said. “If this information is interpreted in the future in terms of dark matter, we made a very, very important discovery. If it is in terms of pulsars, we did a very good experiment.”

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Image: PAMELA