The dynamics of stars, galaxies and clusters of galaxies offer strong
circumstantial evidence for the existence of substantial amounts of
matter that exerts a gravitational influence but which is not apparent
at visible wavelengths.  There have been recent claims for the direct
detection of dark matter: two groups have detected stellar brightenings
toward the Large Magellenic Cloud that they attribute to micro-lensing
of LMC stars by sub-stellar objects in the halo of the Milky Way;
another group claims to have detected faint light in a halo around
NGC5907 that follows the large scale needed to produce  the flat
rotation curve seen in this galaxy.  But direct results are few and
arguments continue about the nature of the dark matter. ISO
observations have a fundamental role to play in   attacking this
problem.  Perhaps the last refuge for  baryonic dark matter is faint
red stars, at or below the low mass cutoff of the  main sequence.  A
cloud of  stars or brown dwarfs with masses less than a few tenths of a
solar mass could have escaped detection at visible wavelengths yet, if
sufficient in number, could account for the missing halo matter.
Detection of the dark matter halo by ISO would be a fundamental
contribution to astrophysics.  This proposal outlines an approach that,
while at the limit of ISO's capabilities, will either detect a dark
matter halo or set a useful upper limit to its content.
Finally, it is worth pointing out that these observations will
result in a deep survey of about 0.07 sq.deg of high latitude
sky at 4.5 um. The statistics of the star and galaxy counts at
this flux level will be of interest by themselves.