The gravitational pull of cold dark matter in galaxy clusters can distort or bend the light coming from distant background galaxies, in a phenomenon called gravitational lensing. The pull of small concentrations of cold dark matter embedded within a galaxy cluster can modify the overall distribution of this gravitational lensing. In a new study of observations of 11 galaxy clusters, Massimo Meneghetti and colleagues find that galaxy clusters contain far more of these smaller lenses--an order of magnitude more--than expected from standard cosmology. Meneghetti et al. compared predictions of small-scale lensing effects, from simulations of the structure of galaxy clusters, with observations of lensing collected using the Hubble Space Telescope and the Very Large Telescope. They find far more small lenses than expected from the simulations. The researchers conclude that there must be either an unknown problem with standard simulation methods, or that cosmologists have made incorrect assumptions about the nature of dark matter itself.