  Lensing flux-ratio anomalies are most likely caused by gravitational lensing
by small-scale dark matter structures. These anomalies offer the prospect of
testing a fundamental prediction of the cold dark matter (CDM) cosmological
model: the existence of numerous substructures that are too small to host
visible galaxies. In two previous studies we found that the number of subhalos
in the six high-resolution simulations of CDM galactic halos of the Aquarius
project is not sufficient to account for the observed frequency of flux ratio
anomalies seen in selected quasars from the CLASS survey. These studies were
limited by the small number of halos used, their narrow range of masses (1-2E12
solar masses) and the small range of lens ellipticities considered. We address
these shortcomings by investigating the lensing properties of a large sample of
halos with a wide range of masses in two sets of high resolution simulations of
cosmological volumes and comparing them to a currently best available sample of
radio quasars. We find that, as expected, substructures do not change the
flux-ratio probability distribution of image pairs and triples with large
separations, but they have a significant effect on the distribution at small
separations. For such systems, CDM substructures can account for a substantial
fraction of the observed flux-ratio anomalies. For large close-pair separation
systems, the discrepancies existing between the observed flux ratios and
predictions from smooth halo models are attributed to simplifications inherent
in these models which do not take account of fine details in the lens mass
distributions.
