  We examine whether a cosmologically significant distribution of dark galaxy
group or cluster-sized objects can have an optical depth for multiple imaging
of distant background sources which is comparable to that from known galaxies
while at the same time producing angular splittings of the same order of
magnitude. Our purpose is to explore whether such objects could realistically
account for some of the observed lenses. Modeling such systems as isothermal
spheres with core radii, and assuming a Schechter-type distribution function,
we find that independent of the cosmology (open, flat matter dominated, or flat
cosmological constant dominated) an allowed parameter range exists which is
comparable in velocity dispersion to that for known compact groups of galaxies,
although the preferred core radii are somewhat smaller than that normally
assumed for compact groups. Dark cluster-sized objects, on the other hand,
cannot reproduce the observed lensing characteristics. If the one known Dark
cluster were a good representative of such a distribution, most such objects
would not produce multiple images. We also present a result for the angular
splitting due to an isothermal sphere lens with non-zero core radius, extending
earlier work of Hinshaw and Krauss (1987). Our results are expressed as contour
plots for fixed lensing probabilities, and angular splittings.
