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A rotation curve (or galaxy rotation curve) of a galaxy is a function over its radial dimension mapping to the orbital speed of the stars at that radius. Rotation curves are of interest in that they do not show the pattern expected from Keplerian orbits based upon the mass observed in the galaxy's stars and clouds. This mismatch is a primary motivation for the concept of dark matter.
The rotation curve of the Milky Way is the primary factor in the concept of kinematic distance.