  Models in which a large fraction of the Galactic dark matter takes the form
of cold gas clouds imply that there is thermal microwave emission from the
Galactic dark halo. Such models can therefore be directly constrained by data
on the microwave sky, and in particular the very sensitive observations of
microwave anisotropies which are now being made. To this end we have computed
the anisotropy power-spectrum expected for a Galactic dark halo made of cold,
dense gas clouds, including the effects of clustering with a CDM-like mass
spectrum of mini-halo substructure. The power-spectrum displays two peaks: one,
at l~50, is the Poisson noise for the mini-halos, and the second, much larger
and at much higher l, is the Poisson noise of the individual clouds. Because it
appears on small (milli-arcsecond) angular scales, where the instrumental
sensitivity is inevitably very poor, the latter signal is not directly
detectable. By contrast, clusters of cold gas clouds may contribute
significantly to the observed anisotropies if their emission has a grey-body
spectrum. In this case the peak fluctuation, at l~50, amounts to 4/|sin b|
micro-K in the Rayleigh-Jeans limit, and is the dominant Galactic foreground
between 40 and 80 GHz. It will be possible to constrain this foreground
component using low-latitude data from the MAP satellite, providing that its
spectrum conforms to a grey-body. If the spectrum is ``dusty'' there will be
relatively little power at frequencies below the thermal peak, and in this case
the predicted anisotropies are shown to be negligible.
