  We study gravitational lensing by clusters of galaxies in the context of the
generic class of unconventional gravity theories of the scalar--tensor type.
For positive energy scalar fields with any dynamics, the bending of light by a
weakly gravitating system (galaxy or cluster) is always smaller than the
bending predicted by general relativity for the mass of visible and hitherto
undetected matter (but excluding the scalar field energy). The same conclusion
obtains within general relativity if a nonnegligible part of the mass in
clusters is in coherent scalar fields, {\it i.e.\/} Higgs fields. Thus the
observational claim that clusters of galaxies deflect light much more strongly
than would be expected from the observable matter contained by them, cannot be
interpreted in terms of some scalar-tensor unconventional gravity theory with
no dark matter. If the observations eventually show that the matter
distribution inferred via general relativity from the lensing is very much like
that determined from the dynamics of test objects, then scalar--tensor
unconventional gravity will be irrelevant for understanding the mass
discrepancy in clusters. However, even a single system in which the dynamically
determined mass significantly exceeds the lensing mass suggested by general
relativity, would be very problematic for the dark matter picture, but would be
entirely consistent with unconventional scalar--tensor gravity.
