Summary

Extreme scattering events reveal the presence of compact, high pressure regions of ionised gas in the Galaxy; these are best explained in terms of photo-ionisation of cold clouds, but these clouds must contribute substantially to the Galactic dark matter. This interpretation involves no incompatibility with other data and most of its challenges are theoretical -- i.e. the need to understand the origin, structure and evolution of the clouds. Model predictions to date have been in remarkable accord with observation; in particular the Tully-Fisher relation for spiral galaxies emerges as a natural consequence of the model.

The most controversial arena for the cold cloud model at present is cosmology, where the view that most of the dark matter is non-baryonic is usually rather firmly held. Nevertheless one can sensibly contemplate a purely baryonic universe if the baryons are inhomogeneously distributed at nucleosynthesis, and these inhomogeneities may be identified with the proto-clouds themselves. Consistency with the observed large-scale CMB anisotropies then requires either isothermal primordial density fluctuations or, if the fluctuations are adiabatic, early decoupling of the clouds from the CMB.

The cold cloud model still requires a great deal of theoretical development and observational investigation; in particular we need a definitive observational test of the existence of the putative clouds.

© Copyright Astronomical Society of Australia 1997