  It has been suggested that the dark halos of galaxies are constituted by
cloudlets of cold (< 10 K) H_2 and dense (> 10^7$ cm^{-3}) molecular gas. Such
gas is extremely difficult to detect, because the classical tracers of
molecular gas, CO and/or dust grains, have very low abundances and their
emission is exceedingly weak. For this reason, the cloudlet hypothesis remains
so far substantially unproven. In this Letter we propose a new method to probe
the presence of cold H_2 clouds in galactic halos: the ground transition of
ortho-H_2D^+ at 372 GHz. We discuss why the H_2D^+ is abundant under the
physical conditions appropriate for the cloudlets, and present a chemical model
that predicts the H_2D^+ abundance as function of four key parameters: gas
density and metallicity, cosmic ray ionization rate and dust grain size. We
conclude that current ground-based instruments might detect the ortho-H_2D^+
line emitted by the cloudlets halo, and prove, therefore, the existence of
large quantities of dark baryonic matter around galaxies.
