  Several lines of evidence suggest that cold, dense gas clouds make a
substantial contribution to the total mass of dark halos. If so then physical
collisions between clouds must occur; these cause strong, radiative shocks to
propagate through the cold gas, with the startling implication that all
``dark'' halos must be luminous. The expected luminosity is a strong function
of halo velocity dispersion, and should contribute a significant fraction of
the observed X-ray emission from clusters of galaxies, if dark halos are
predominantly made of cold gas.
  Existing data do not exclude this possibility; indeed two particular
expectations of the luminous-halo model are borne out in the X-ray data, and
thus give support to the cold-cloud dark matter model. First we find a
luminosity-temperature correlation of the form L proportional to T to the power
11/4, as seen in recent analyses of cluster samples. Secondly the anticipated
spectra have substantially more power at low energies than isothermal
bremsstrahlung spectra, and might account for the observed ``excess'' EUV
emission seen from some clusters. The successes of the luminous-halo model are
particularly remarkable because the theory has no free parameters or ad hoc
elements. The model can be tested by the X-ray satellite Chandra, which should
resolve the Virgo cluster into 10,000 point-like, transient X-ray sources.
Non-detection of any such sources by Chandra can constrain the contribution of
cold gas clouds to below 1% of the total matter density in the Universe,
assuming Virgo to be representative.
