An experiment that dangled from a balloon high above the Antarctic may have found the most convincing evidence yet that dark-matter particles are annihilating within our own galaxy. Indeed, Earth could even be whizzing through a clump of annihilating dark matter right now.

The Advanced Thin Ionization Calorimeter (ATIC) detected an excess of high-energy cosmic ray electrons in the 300–800 GeV range — an unexpected feature that could be caused by the annihilation of weakly interacting massive particles (WIMPs) — one of the leading candidates for dark matter. If confirmed, the signal could be the first direct detection of dark matter and help physicists understand the nature and origin of this mysterious stuff that appears to make up about 22% of the mass of the universe.

The discovery comes hot on the heels of a similar report of an excess of cosmic-ray positrons by the PAMELA experiment, which could also be a signature of WIMP annihilation.

In either case, we believe this is an unexpected and exciting result John Wefel, principal investigator for ATIC

According to the international team of scientists running ATIC, the excess of cosmic electrons cannot be explained by the standard model of cosmic ray origin and propagation in the galaxy. Instead, it suggests the existence of a hitherto unknown and nearby source of high-energy electrons (Nature 456 362).

WIMPs or a new astronomical object?

"We have possibly either the first detection of a nearby source of particle acceleration from an as yet unstudied object, or the detection of the signature of predicted dark matter particle annihilation,” explains John Wefel at Louisiana State University in the US. Wefel, who is principal investigator for the ATIC experiment added, “In either case, we believe this is an unexpected and exciting result.”

Indeed, the team was not searching specifically for dark matter — ATIC was launched in 2002 to gain a better understanding of the very-high energy cosmic ray electron spectrum.

The researchers had expected to see electrons from unknown sources outside our galaxy and those cosmic ray electrons produced closer to Earth in the interstellar medium by the interactions of cosmic ray protons and helium nuclei with interstellar gas. Instead, they found something very different.

Mysterious 'bump'

Their measured spectrum followed the predicted spectrum up to about 100 GeV but then began to rise to produce a distinct ‘bump’ between about 300 and 800 GeV, far exceeding the predicted flux of electrons, before having a cliff-like drop-off to return back to the predicted spectrum above 800 GeV.

What is unique is that it is the first time a feature of this kind has been discovered in the cosmic ray electron spectrum James Adams, NASA

“This bump was not expected and at its peak, the excess flux of electrons is about three times above the predicted flux,” says James Adams, a member of the team at NASA's Marshall Space Flight Center in Alabama. “What is unique is that it is the first time a feature of this kind has been discovered in the cosmic ray electron spectrum and it cannot be explained by the standard model of cosmic ray origin and propagation in the galaxy.”

One possible explanation may be that the bump is due to a strong nearby cosmic ray electron source, such as a supernova remnant, a pulsar wind nebula or an intermediate-mass black hole. However, no suitable nearby object is known to exist.

A more intriguing alternate explanation is that the source is caused by the annihilation of dark matter particles. Scientists now believe dark matter makes up the bulk of all matter in the universe, outnumbering normal matter by 5 to 1. However, its nature remains a mystery. It is fundamentally different from normal “luminous” matter such as stars as it is invisible to modern telescopes, giving off no light or heat, and interacts only through gravity, making it difficult to detect.

Physicists believe dark matter particles such as WIMPs can collide and annihilate each other, producing electron-positron pairs and emitting tell-tale electron patterns that could be detected by space-based telescopes.

Kaluza-Klein a likely candidate

“There is such a candidate dark matter particle, the Kaluza-Klein particle,” explains Adams. “The electron spectrum from the annihilation of Kaluza-Klein particles fits the measured bump well, if we assume that there is a local clump of Kaluza-Klein particles with a density about 200 times the galactic average.”

Other cosmologists are excited by the finding. “The possibility that we may be seeing the products of dark matter annihilation within a short distance from the Sun is very exciting,” says Piero Madau at the University of California, Santa Cruz.

The possible dark matter interpretation is exciting, but it may be quite some time before the smoke clears on this and a consensus emerges Stephane Coutu, Pennsylvania State University

Others are more cautious. “The electron excess they observe is intriguing as cosmic electrons must come from relatively local sources, but what these sources are is very speculative,” says Stephane Coutu at Pennsylvania State University. “The possible dark matter interpretation is exciting, but it may be quite some time before the smoke clears on this and a consensus emerges.”

Meanwhile, there are other hints of interesting signals, for instance in the recent cosmic positron measurements by the PAMELA satellite experiment. While everyone isn’t sure yet whether the two results are fully consistent with each other, both seem to indicate either "new physics" or "new astrophysics" in the 100–500 GeV region of the cosmic-ray spectrum.

“We will have to see what other instruments observe, such as the Fermi satellite project now in operation to detect energetic cosmic gamma rays,” says Coutu. “Only through a convergence of many separate independent hints will the interpretation crystallize.”