A Speed Test for Dark Matter

Dark matter dominates the large-scale dynamics of the Universe, but what it is remains a puzzle. Recent observations of an unexplained x-ray line in the emission from our Galaxy, as well as distant galaxies, suggest we might have detected—at least indirectly—the particles that make up dark matter (see 15 December 2014 Viewpoint). But so far, astrophysicists haven’t been able to rule out the possibility that the spectral line comes from ordinary gas. A team of scientists now suggests the signal’s origin could be pinned down by observing the emission from our own galaxy and analyzing how the line’s frequency shifts when it is observed along different directions with respect to the Galactic Center [1]. The sign of the shift (to higher or lower frequencies) is different for dark matter and ordinary gas particles because of their distinct velocity distributions. Although these frequency changes are too small to detect with current x-ray spectrometers, they should be observable by upcoming missions.