
%0 Journal Article
%A Wardle, Mark 
%A Walker, Mark 
%T The Cloudy Universe
%J Publications of the Astronomical Society of Australia
%D 1999/11/01
%V 16
%N 3
%P 262-272
%K 
%R https://doi.org/10.1071/AS99262
%U https://www.publish.csiro.au/paper/AS99262 
%X 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 Halpha emission from clouds in the solar neighbourhood, optical and X-ray flashes arising from cloud-cloud collisions, ultraviolet extinction, and three varieties of lensing phenomena. 
