  The distortion of the images of faint high-redshift galaxies can be used to
probe the intervening mass distribution. This weak gravitational lensing effect
has been used recently to study the (projected) mass distribution of several
clusters at intermediate and high redshifts. In addition, the weak lensing
effect can be employed to detect (dark) matter concentrations in the Universe,
based on their mass properties alone. Thus it is feasible to obtain a
mass-selected sample of `clusters', and thereby probe the full range of their
mass-to-light ratios. We study the expected number density of such haloes which
can be detected in ongoing and future deep wide-field imaging surveys, using
the number density of haloes as predicted by the Press-Schechter theory, and
modeling their mass profile by the `universal' density profile found by
Navarro, Frenk & White. We find that in all cosmological models considered, the
number density of haloes with a signal-to-noise ratio larger than 5 exceeds 10
per square degree. With the planned MEGACAM imaging survey of $\sim 25 deg^2$,
it will be easily possible to distinguish between the most commonly discussed
cosmological parameter sets.
