galactic rotation
Rotation curve of the spiral galaxy M33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.
Galactic rotation is the systematic, differential rotation of stars and interstellar matter in a galaxy about an axis that passes through the galactic center. Spiral and lenticular galaxies have a well-defined rotational structure whereas the individual stars in elliptical galaxies tend to move in random orbits.
