Weak Gravitational Lensing by Dark Clusters
  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. 
Nevin N. Weinberg, Marc Kamionkowski (Caltech)