Galactic cosmic rays — high-energy charged particles that result from hugely energetic processes — are one of the few ways to directly measure matter from outside the Solar System. In 2000, Jin Chang, now an astrophysicist at the Purple Mountain Observatory in Nanjing, China, and a group of international colleagues first detected an excess of high-energy cosmic-ray electrons using the Advanced Thin Ionization Calorimeter (ATIC). The instrument was sent on helium balloons to measure the composition and energy spectra of cosmic rays 35 kilometres above Antarctica. Potential sources of such high-energy electrons could be a pulsar, a supernova or an intermediate-mass black hole. But the existing data don't confirm or refute any of these. In fact, Chang tells Nature, this could be the first indirect evidence of dark matter, the theoretical matter thought to make up as much as 85% of matter in the Universe (see page 362).
Why did it take so long to publish this work?
During an ATIC flight in 2000–01, we were surprised to find an excess of high-energy (∼300–800 gigaelectronvolt) electrons — observations that had to be verified to be believed. But balloon observations are very difficult to make. In fact, balloons we deployed with improved instrumentation to reduce background noise during the third flight malfunctioned. We had to wait another two years to try again. In the end, the second and fourth flights — as well as measurements from Japan's Polar Patrol Balloon — successfully verified the pattern of high-energy electrons.
Of all the possible sources, which do you think is most likely?
Our data suggest that the source could be an unknown nearby astrophysical object capable of accelerating electrons to this energy level — such as a pulsar, a rapidly spinning star that produces regular pulses of radiation, or a supernova, the explosive death of a massive star. But they also don't discount that the electrons could be produced by the annihilation of dark-matter particles. We have a long way to go to definitively determine the source.
Do you think that we will understand dark matter within the next decade?
I hope so. The ATIC team plans to build a new instrument to study this source further. We are also waiting for new results from several space-based instruments. The Large Hadron Collider, the new particle accelerator at CERN near Geneva, Switzerland, may also contribute by creating and detecting predicted dark-matter particles.