What I have learned about the Milky Way’s dynamics from Gaia so far

The ESA Gaia mission is an astrometric mission to map over a billion stars in the Milky Way in 3D position, 3D velocity, and more. Since its first data release in September 2016 and especially since its recent second data release April 2018, Gaia has impacted nearly every field in astrophysics. Nowhere is this more the case than in the study of Milky-Way dynamics. I will describe new insights into the dynamics of the Milky Way gleaned from Gaia data, including the origin of the Milky Way’s bar and spiral structure and the first evidence of the presence of a population of dark subhalos predicted by the cold-dark-matter paradigm.