  We have recently proposed a new candidate for baryonic dark matter: very cold
molecular gas, in near-isothermal equilibrium with the cosmic background
radiation at 2.73 K. The cold gas, of quasi-primordial abundances, is condensed
in a fractal structure, resembling the hierarchical structure of the detected
interstellar medium.
  We present some perspectives of detecting this very cold gas, either directly
or indirectly. The H$_2$ molecule has an "ultrafine" structure, due to the
interaction between the rotation-induced magnetic moment and the nuclear spins.
But the lines fall in the km domain, and are very weak. The best opportunity
might be the UV absorption of H$_2$ in front of quasars. The unexpected cold
dust component, revealed by the COBE/FIRAS submillimetric results, could also
be due to this very cold H$_2$ gas, through collision-induced radiation, or
solid H$_2$ grains or snowflakes. The $\gamma$-ray distribution, much more
radially extended than the supernovae at the origin of cosmic rays
acceleration, also points towards and extended gas distribution.
