Satellite Galaxies, also called Dwarf Galaxies, were small galaxies that orbited larger galaxies. The contained many stars, but not as many as a larger galaxy.
Mutter's Spiral had a Satellite Galaxy called Galaxy Seven, which was likely its seventh satellite galaxy. (NSA: The Eyeless).
In 4126, the Tenth Doctor and Donna Noble were shown a map of the Tri-Galactic that was controlled by the Second Great and Bountiful Human Empire, and it likely included satellite Galaxies and the Andromeda Galaxy and possibly others as well. (DW: Planet of the Ood)
In the 43rd century, Humans colonised the Canis Major Dwarf galaxy, likely another satellite of Mutter's Spiral. (DWBIT: #45)