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.
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