Dark matter in spiral galaxies

The Tully-Fisher relation is used to probe dark matter (DM) in the optical regions of spiral galaxies. By establishing it at several different isophotal radii in an appropriate sample of 58 galaxies with good B-band photometry and rotation curves, we find that some of its attributes such as scatter, residuals, nonlinearity, and bias dramatically decrease moving from the disk edge inward. We show how this behavior challenges any mass model which assumes no DM or a luminosity-independent DM mass fraction interior to the optical radius of spiral galaxies.