First Results on Dark Matter Substructure from Astrometric Weak Lensing.

Low-mass structures of dark matter (DM) are expected to be entirely devoid of light-emitting regions and baryons. Precisely because of this lack of baryonic feedback, small-scale substructures of the Milky Way are a relatively pristine testing ground for discovering aspects of DM microphysics and primordial fluctuations on subgalactic scales. In this Letter, we report results from the first search for Galactic DM subhalos with time-domain astrometric weak gravitational lensing.