Black Holes and Revelations: Unseen Companions in Stellar Binaries

The Milky Way contains of order 100 million stellar-mass black holes. Yet, fewer than 100 black hole candidates are known in the Milky Way, and only about 25 are dynamically confirmed. For the last 50 years, our view of the Milky Way's black hole population has been shaped almost entirely by observations of X-ray binaries, which shine brightly but represent an extremely rare outcome of binary evolution. The supremacy of X-ray binaries is about to end: the 4th data release of the Gaia mission -- this December! -- is expected to uncover a population of non-interacting black holes that is larger than the X-ray binary population, has a better-understood selection function, and probes a different regime of stellar and binary evolution. I will discuss the tip of the iceberg that has already been revealed by astrometry from Gaia DR3, upcoming opportunities from DR4, and our evolving view of the compact object population. I will highlight constraints on the stability and efficiency of mass transfer from au-scale binaries, which astrometry is particularly sensitive to.