Cold Molecular Gas as Baryonic Dark Matter

We review the different cold dark gas models that have been proposed in the literature, as well as a new variant which addresses their principal stability problems by taking into account the property of molecular hydrogen to become solid or liquid below 33 K and at sufficiently high pressure. This new physical ingredient provides the possibility to stabilise cold gas globules by a core of condensed molecular hydrogen. Such loosely bound cold globules behave in a galaxy as a collisionless ensemble of matter, and form a reservoir of gas easily liberated through, e.g., UV excitation. the cold condensed cores survive the longest, of order a Gyr in the solar neighbourhood radiation field, and much longer in spiral outer HI disks.