  Cold dark matter (CDM) constitutes most of the matter in the Universe. The
interplay between dark and luminous matter in dense cosmic environments like
galaxy clusters is studied theoretically using cosmological simulations.
Observed gravitational lensing is used to test and characterize the properties
of substructures - the small-scale distribution of dark matter - in clusters.
An apt metric, the probability of strong lensing events produced by dark matter
substructure, is devised and computed for 11 galaxy clusters. We report that
observed cluster substructures are more efficient lenses than predicted by CDM
simulations, by more than an order of magnitude. We suggest that hitherto
undiagnosed systematic issues with simulations or incorrect assumptions about
the properties of dark matter could explain our results.
