Globular clusters are large congregations of stars, which gradually lose their members to form thin and long stellar streams. In pristine conditions, these streams have a nearly uniform density, however, new observations of one such structure in the Milky Way halo have revealed a likely site of perturbation. The on-sky morphology suggests a recent, close encounter with a massive and dense perturber. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass clump of dark matter as a plausible candidate. This discovery opens up the exciting possibility that detailed observations of streams could measure the abundance of dark-matter substructure and thus shed light on the nature of dark matter.
