1 99 9 The Cloudy Universe

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 Hα emission from clouds in the solar neighbourhood, optical flashes arising from cloud-cloud collisions, ultraviolet extinction, and three varieties of lensing phenomena.