Galactic rotation curves plot a galaxy's circular velocity (which can be measured using the Doppler shift of HI regions of the trailing and leading sides as viewed from the Earth) vs. the distance from the center of rotation. The fact that rotation curves flatten out at large distances instead of falling off indicates that a substantial amount of mass must exist which is not directly observable (i.e., luminous). Rotation curves are often empirically fit assuming a spherical halo with