  The best measured rotation curve for any galaxy is that of the dwarf
spiralXXXX DDO 154, which extends out to about 20 disk scale lengths. It
provides an ideal laboratory for testing the universal density profile
prediction from high resolution numerical simulations of hierarchical
clustering in cold dark matter dominated cosmological models. We find that the
observed rotation curve cannot be fit either at small radii, as previously
noted, or at large radii. We advocate a resolution of this dilemma by
postulating the existence of a dark spheroid of baryons amounting to several
times the mass of the observed disk component and comparable to that of the
cold dark matter halo component. Such an additional mass component provides an
excellent fit to the rotation curve provided that the outer halo is still cold
dark matter-dominated with a density profile and mass-radius scaling relation
as predicted by standard CDM-dominated models. The universal existence of such
dark baryonic spheroidal components provides a natural explanation of the
universal rotation curves observed in spiral galaxies, may have a similar
origin and composition to the local counterpart that has been detected as
MACHOs in our own galactic halo via gravitational microlensing, and is
consistent with, and even motivated by, primordial nucleosynthesis estimates of
the baryon fraction.
