  We explore some basic observational consequences of assuming that the dark
matter in the Milky Way consists mainly of molecular clouds, and that cosmic
rays can penetrate these clouds. In a favoured model of the clouds, this
penetration would have the following consequences, all of which agree with
observation:
  (i) cosmic ray nuclei would be fragmented when they enter a cloud, giving
them a lifetime in the Galaxy of $10^{15}$ sec (for relativistic nuclei).
  (ii) Pionic $\gamma$ rays emitted by the clouds, after pp collisions, would
have a flux in the galactic plane comparable to the flux from known sources for
photon energies $> \sim 1$ Gev.
  (iii) The heat input into the clouds from cosmic rays would be re-radiated
mainly in the far infra-red. The resulting radiation background agrees, in both
intensity and spectrum in different directions, with a known excess in the far
$i - r$ background of the galaxy over emission by warm dust.
