  Ultra compact dwarf galaxies (UCDs) are dense stellar systems at the border
between massive star-clusters and small galaxies. Their on average high optical
mass-to-light (M/L) ratio cannot be explained by stellar populations with the
canonical stellar initial mass function (IMF), while it is doubtful that
non-baryonic dark matter can accumulate enough on the scales of UCDs for
influencing their dynamics significantly. UCDs in the Virgo galaxy cluster
apparently also have an over-abundance of neutron stars, strongly suggesting a
top-heavy IMF, which would explain both findings. This is because a top-heavy
IMF can provide the unseen mass through an abundance of stellar remnants. The
suggested variation of the IMF can be understood if UCDs represent a case of
rapid star-formation in an extremely dense environment. While top-heavy IMFs
imply a much heavier mass-loss shortly after the formation of a stellar system,
this process does not necessarily dissolve the UCDs. Their formation with a
top-heavy IMF would therefore not contradict their existence.
