The observed variation in orbital speed of stars and interstellar matter with distance from the centre of a galaxy. The curve is determined by measuring the Doppler shift of spectral lines at optical, infrared, millimetre, or radio wavelengths. The observed ‘flatness’ of these curves at large radii in many spiral galaxies, where the velocities would be expected to decrease with radius, implies the existence of large amounts of unseen dark matter beyond the visible regions.