The Milky Way galaxy has many satellite galaxies orbiting it. The most massive are the Large and Small Magellanic clouds, but there are dozens of smaller galaxies distributed at varying distances from the Milky Way. Because these galaxies can be found as far as 400 kpc from the MW, by studying their motions it is possible to measure the total mass of the Milky Way, a value that is important for understanding how our local group of galaxies formed and for comparison to cosmological simulations.
The dwarf galaxies around orbiting the Milky Way is composed of a much larger fraction of dark matter than compared to the Milky Way. The ultra-faint dwarfs, which have very few stars, are the most dark matter dominated systems we know of today. By studying the orbits of these systems, we can learn about their past and how they may have interacted with the Milky Way.