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Low-mass structures of dark matter (DM) are expected to be entirely devoid oflight-emitting regions and baryons. Precisely because of this lack of baryonicfeedback, small-scale substructures of the Milky Way are a relatively pristinetesting ground for discovering aspects of DM microphysics and primordialfluctuations on subgalactic scales. In this work, we report results from thefirst search for Galactic DM subhalos with time-domain astrometric weakgravitational lensing. The analysis is based on a matched-filter template oflocal lensing corrections to the proper motion of stars in the MagellanicClouds. We describe a data analysis pipeline detailing sample selection,background subtraction, and handling outliers and other systematics. Fortentative candidate lenses, we identify a signature based on an anomalousparallax template that can unequivocally confirm the presence of a DM lens,opening up prospects for robust discovery potential with full time-series data.We present our constraints on substructure fraction $f_l \lesssim 5$ at 90% CL(and $f_l \lesssim 2$ at 50% CL) for compact lenses with radii $r_l <1\,\mathrm{pc}$, with best sensitivity reached for lens masses $M_l$ around$10^7$-$10^8\,M_\odot$. Parametric improvements are expected with futureastrometric data sets; by end of mission, $Gaia$ could reach $f_l \lesssim10^{-3}$ for these massive point-like objects, and be sensitive to lighterand/or more extended subhalos for $\mathcal{O}(1)$ substructure fractions.

First Results on Dark Matter Substructure from Astrometric Weak Lensing