Like some offshore tax shelter, the cosmic matter portfolio is mostly invisible. Theories abound on what this dark matter might be, but the experimental goal now is to find hard evidence of the nature and properties of these particles that can govern the dynamics of galaxies but remain out of sight. Researchers have looked for telltale signs such as gamma rays from dark matter annihilation and perturbations of the cosmic microwave background; is the dark matter made up of WIMPs (weakly interacting massive particles) or primordial black holes or something even more exotic? Search the archives of Physics, and you will be spoiled for choice (see 1 December 2011 Synopsis and 8 December 2011 Synopsis).
Writing in Physical Review Letters, Nicolao Fornengo of the University of Turin, Italy, and colleagues suggest that dark matter might be identified in radio waves emitted from beyond our galaxy. Their idea is based on data collected by the Absolute Radiometer for Cosmology, Astrophysics and Diffusion Emission (ARCADE ) instrument. The ARCADE collaboration found an unusual excess in isotropic radio flux in the frequency window from to gigahertz, once they accounted for known extragalactic sources. Fornengo et al. propose that, because the above-normal flux is difficult to explain in standard astrophysical scenarios, the emission might result from synchrotron radiation produced by secondary particles generated by the decay or annihilation of WIMPs. If it can be confirmed that this is the case, it may lead to the detection of important nongravitational signatures of dark matter, and the forensic bank examiners of astrophysics might have a shot at balancing the cosmic ledger books. – David Voss