Extended dark halos are well established around isolated unperturbed
 spiral galaxies, and may also exist around ellipticals. There is a
 close coincidence between the total mass in spiral halos and the
 total baryonic mass in the Universe, deduced from light element
 nucleosynthesis. Thus, dark halos may be baryonic. The only form for
 this dark baryonic matter which has not been excluded is brown dwarfs
 of a few Jupiter masses. ISO observations of carefully selected
 galaxies will either see this mass, or prove dark matter to be
 non-baryonic. The sample galaxies must be edge-on, have rotation
 curves, be isolated, have no bulge or strong thick disc, and
 preferably be of high enough redshift so their angular size is
 well matched to the ISOCAM field of view. One may then fit
 dynamically-allowed surface density distributions to the surface
 brightness data for several galaxies, to search for consistent
 results. We have isolated such a sample, and will use ISOCAM to
 detect or disprove the existence of brown dwarfs as the explanation
 of dark matter halos in spiral galaxies.