The cloudy universe

Mark Walker, Mark Wardle

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


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 Ha 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.

Original languageEnglish
Pages (from-to)262-272
Number of pages11
JournalPublications of the Astronomical Society of Australia
Issue number3
Publication statusPublished - 1999
Externally publishedYes


  • dark matter
  • galaxies: halos
  • ISM: clouds


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