  Modelling of Extreme Scattering Events suggests that the Galaxy's dark matter
is an undetected population of cold, AU-sized, planetary-mass gas clouds. None
of the direct observational constraints on this picture -- thermal/non-thermal
emission, extinction and lensing -- are problematic. The theoretical situation
is less comfortable, but still satisfactory. Galactic clouds can survive in
their current condition for billions of years, but we do not have a firm
description for either their origin or their evolution to the present epoch. We
hypothesise that the proto-clouds formed during the quark-hadron phase
transition, thereby introducing the inhomogeneity necessary for compatibility
with light element nucleosynthesis in a purely baryonic universe. We outline
the prospects for directly detecting the inferred cloud population. The most
promising signatures are cosmic-ray-induced H-alpha emission from clouds in the
solar neighbourhood, optical flashes arising from cloud-cloud collisions,
ultraviolet extinction, and three varieties of lensing phenomena.

Astrophysics