The Gaia Mission is making the most precise measurements ever made of stars. These measurements have the promise to resolve physics issues with the Solar System, stars, and the Milky Way, among others. One hope is that the Mission's kinematic measurements might give us an image of the dark matter in the Milky Way or produce results that identify the detailed physical characteristics of the dark matter particle. I will explain how the Mission works, and how it might deliver on these hopes. I will also show evidence we have already found in the data for a dark-matter substructure orbiting in the Milky Way halo.