Direct detection of cold dark matter substructure

We devise a method to measure the abundance of satellite halos in gravitational lens galaxies and apply our method to a sample of seven lens systems. After using Monte Carlo simulations to verify the method, we find that substructure comprises f(sat) = 0.02 (median, 0.006 < f(sat) < 0.07 at 90% confidence) of the mass of typical lens galaxies, in excellent agreement with predictions of cold dark matter (CDM) simulations. We estimate a characteristic critical radius for the satellites of 0.0001 < b < 0.006 (90% confidence). For a dn/dM proportional to M-1.8 (M-low < M < M-high) satellite mass function, the critical radius provides an estimate for the upper mass limit of 10(6) M-. less than or similar to M-high less than or similar to 10(9) M-.. Our measurement confirms a generic prediction of CDM models and may obviate the need to invoke alternatives to CDM such as warm dark matter or self-interacting dark matter.