Abstract
A fraction of dark matter may be in the form of cold, primordial fractal clouds as proposed by Pfenniger, Combes, & Martinet. Far from sources of heating, the gas is thermalized to 3 K, but there are regimes where the cloudlets collide. A scenario is proposed where the smooth neutral hydrogen in spiral galaxies is produced when cloudlets annihilate through collisions. In this scheme, the cold gas does not collapse by energy dissipation, but instead the collision energy warms up the gas trapped in the cloudlets, which evaporates. It is shown that the evaporated neutral hydrogen distribution is not proportional to the dark gas distribution, but a relation between the two may in principle be recovered from the equation of annihilation and from the dissipative collapse of the neutral hydrogen.
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