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 haloes containing identifiable X-ray and/or optical clusters. However, a significant fraction are non-virialized, cluster-mass 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 is consistent with the abundance implied by the detection by Erben et al. of an apparent dark lens. Weak lensing by these non-virialized 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.