  We construct an all-sky map of the apparent temperature and optical depth of
thermal dust emission using the Planck-HFI and IRAS data. The optical depth
maps are correlated to tracers of the atomic and molecular gas. The correlation
is linear in the lowest column density regions at high galactic latitudes. At
high NH, the correlation is consistent with that of the lowest NH. In the
intermediate NH range, we observe departure from linearity, with the dust
optical depth in excess to the correlation. We attribute this excess emission
to thermal emission by dust associated with a Dark-Gas phase, undetected in the
available HI and CO measurements. We show the 2D spatial distribution of the
Dark-Gas in the solar neighborhood and show that it extends around known
molecular regions traced by CO. The average dust emissivity in the HI phase in
the solar neighborhood follows roughly a power law distribution with beta = 1.8
all the way down to 3 mm, although the SED flattens slightly in the millimetre.
The threshold for the existence of the Dark-Gas is found at NH = (8.0\pm 0.58)
10^{20} Hcm-2. Assuming the same dust emissivity at high frequencies for the
dust in the atomic and molecular phases leads to an average XCO = (2.54\pm0.13)
10^{20} H2cm-2/(K km s-1). The mass of Dark-Gas is found to be 28% of the
atomic gas and 118% of the CO emitting gas in the solar neighborhood. A
possible origin for the Dark-Gas is the existence of a dark molecular phase,
where H2 survives photodissociation but CO does not. The observed transition
for the onset of this phase in the solar neighborhood (AV = 0.4 mag) appears
consistent with recent theoretical predictions. We also discuss the possibility
that up to half of the Dark-Gas could be in atomic form, due to optical depth
effects in the HI measurements.
