  The cold dark matter (CDM) scenario for structure formation in the Universe
is very attractive and has many successes; however, when its spectrum of
density perturbations is normalized to the COBE anisotropy measurement the
level of inhomogeneity predicted on small scales is too large. This can be
remedied by a tau neutrino of mass $1\MeV - 10\MeV$ and lifetime $0.1\sec -
100\sec$ whose decay products include electron neutrinos because it allows the
total energy density in relativistic particles to be doubled without
interfering with nucleosynthesis. The anisotropies predicted on the degree
scale for ``$\tau$CDM'' are larger than standard CDM. Experiments at $e^\pm$
colliders may be able to probe such a mass range.
