  The severity of the mass discrepancy in spiral galaxies is strongly
correlated with the central surface brightness of the disk. Progressively lower
surface brightness galaxies have ever larger mass discrepancies. No other
parameter (luminosity, size, velocity, morphology) is so well correlated with
the magnitude of the mass deficit.
  The rotation curves of low surface brightness disks thus provide a unique
data set with which to probe the dark matter distribution in galaxies. The mass
discrepancy is apparent from $R = 0$ giving a nearly direct map of the halo
mass distribution. The luminous mass is insignificant.
  Interpreting the data in terms of dark matter leads to troublesome
fine-tuning problems. Different observations require contradictory amounts of
dark matter. Structure formation theories are as yet far from able to explain
the observations.
