  We calculate the abundance of dark-matter concentrations that are
sufficiently overdense to produce a detectable weak-gravitational-lensing
signal. Most of these overdensities are virialized halos containing
identifiable X-ray and/or optical clusters. However, a significant fraction are
nonvirialized overdensities still in the process of gravitational
collapse--these should produce significantly weaker or no X-ray emission. Our
predicted abundance of such dark clusters are consistent with the abundance
implied by the Erben et al. (2000) detection of a dark lens. Weak lensing by
these nonvirialized objects will need to be considered when determining
cosmological parameters with the lens abundance in future weak-lensing surveys.
Such weak lenses should also help shed light on the process of cluster
formation.
