To address, among other questions, puzzling observations about star forming in the extreme outer HI disk of M31 (Cuillandre et al. 2001), a scenario of interstellar gas cycling between the visible and a very cold invisible phase is investigated. The key new element sketched here, allowing to maintain the bulk of the gas out of sight, is that molecular hydrogen becomes liquid or solid below 33 K at sufficiently high pressure, allowing AU-sized spheres of very cold gas to be stabilised by incompressible cores of condensed H2 (Pfenniger 2004). These predicted cold gas globules are relatively weakly bound (~ 10−3eV/nucleon), such that their lifetime depends directly on the ambient UV/CR excitation level. At galactic scale the globules behave as collisionless bodies, and evaporate and become the usual visible ISM gas through heating. Much of the ISM gas can thus spend a long time in this cold condensed phase in low excitation regions. N-body simulations of galactic disks modeling such effects have been run, and some of their features are described in more detail in this volume and elsewhere (Revaz & Pfenniger 2004).