Dark matter clump spotted in a distant gravitational lens

Using ALMA's sharpest millimeter observations of a strongly lensed galaxy, astronomers found unmistakable evidence for a dark matter subhalo embedded in the lens. The subhalo has a mass of roughly 28 billion suns and shows concentration values typical of N-body simulations. This detection matters because cold and warm dark matter models predict wildly different numbers of such substructures—a direct way to distinguish between them.