2.30 pm Monday 10 December 2001
Part of the purported crisis afflicting the Cold Dark Matter (CDM) scenario is the apparent discrepancy between the number of satellite halos predicted by CDM simulations and the observed number of Galactic satellite galaxies. We devise a method to measure the abundance of subhalos in lens galaxies, and apply our method to a sample of 7 lens systems. We find that substructure comprises a significant fraction of the mass of typical lens galaxies, in excellent agreement with results of CDM simulations. This measurement confirms a generic prediction of CDM models, and may obviate the need to invoke alternatives to CDM like warm dark matter or self-interacting dark matter.