  Deep optical/near-IR surface photometry of galaxies outside the Local Group
have revealed faint and very red halos around objects as diverse as disk
galaxies and starbursting dwarf galaxies. The colours of these structures are
too extreme to be reconciled with stellar populations similar to those seen in
the stellar halos of the Milky Way or M31, and alternative explanations like
dust reddening, high metallicities or nebular emission are also disfavoured. A
stellar population obeying an extremely bottom-heavy initial mass function
(IMF), is on the other hand consistent with all available data. Because of its
high mass-to-light ratio, such a population would effectively behave as
baryonic dark matter and could account for some of the baryons still missing in
the low-redshift Universe. Here, we give an overview of current red halo
detections, alternative explanations for the origin of the red colours and
ongoing searches for red halos around types of galaxies for which this
phenomenon has not yet been reported. A number of potential tests of the
bottom-heavy IMF hypothesis are also discussed.
