Signature of a Dark Ultracompact Dwarf Galaxy Transiting the Milky Way Disk

Abstract We report the discovery of a vertical velocity anomaly in the stellar component of the Galactic disk, consistent with the impact of a dark, ultracompact dwarf galaxy (UCD)-sized object plunging into the Milky Way. The anomaly spatially coincides with a suite of gaseous disturbances—including an H I void, a molecular shell (CO 16.134–0.553), and a vertical H I filament—previously interpreted as signatures of a dark matter subhalo (DMSH) collision. Analysis of Gaia DR2 astrometry reveals a statistically significant vertical velocity dip colocated with these features, supporting a dynamical origin. The absence of a luminous source at the filament’s terminus suggests a dark or failed UCD. These results provide rare observational evidence for a low-mass DMSH, with implications for the substructure of dark matter halos and constraints on the ΛCDM model.