  We report preliminary results from a first season of photometric monitoring
of 600 quasars behind the Virgo galaxy cluster with the aim of detecting
microlensing by the cluster dark matter. Our project is sensitive to dark
objects of surface mass densities down to ~20 g/cm^2. We are thus capable of
detecting diffuse objects, such as cold molecular clouds, unlike all Galactic
microlensing surveys whose surface mass density limits are > 10^4 g/cm^2. The
average optical depth to microlensing of quasars through the central 30 sq.
deg. of Virgo is ~1x10^-3. We report a null detection which implies that less
than half the dark matter in Virgo is in objects of mass ~10^-5 solar masses,
of surface mass density > 20 g/cm^2, at 90% confidence.
