QSO strong gravitational lensing and the detection of dark halos

We present recent results concerning the possibility to detect dark satellites around galaxies using QSO strong gravitational lensing. Combining high resolution hydrodynamical simulations of galaxy formation and analytic studies we show that current QSO observations data do not present any evidence for the existence of such satellites. The amount of substructures predicted by CDM within a galaxy size dark matter halo is too low to explain the observed anomalies in the QSO images flux ratio. Nevertheless the fluxes of QSO multiple images can be used to constrain the CDM power spectrum on small scales and test different dark matter candidates. We show that a warm dark matter scenario, with an insufficiently massive particle, fails to reproduce the observational data. Our results suggest a lower limit of few keV (~ 10) for the mass of warm dark matter candidates in the form of a sterile neutrino, in good agreement with previous results coming from Lyman- forest and Cosmic Microwave Background analysis.