Milky Way Rotation & Dark Matter: Physical Mechanisms

A Galactic rotation curve plots orbital speed against distance from the center. Gas clouds, stars, and masers trace rotation across the disk, while satellite galaxies and halo objects extend mass constraints farther outward. The inferred dark-matter halo reaches well beyond the visible disk.

For a nearly spherical mass distribution, orbital speed depends on the mass enclosed within the orbit. A flat rotation curve means enclosed mass continues increasing with radius even where visible light falls off strongly, implying a large extended unseen mass component.

Measurements of neutral hydrogen, molecular masers, stellar velocities, satellite motions, and local stellar dynamics all require more gravitating mass than can be supplied by observed baryonic matter alone.

For a roughly spherical mass distribution, flat orbital speed implies enclosed mass continues increasing with radius. A luminous disk alone would predict a stronger decline outside most visible mass.