The properties of multiple image gravitational lenses require a fractional surface mass density in satellites of f=0.02 (0.006 < f < 0.07 at 90% confidence) that is consistent with the expectations for CDM. The characteristic satellite mass scale, 10^6-10^9 Msun, is also consistent with the expectations for CDM. The agreement between the observed and expected density of CDM substructure shows that most low mass galactic satellites fail to form stars, and this absence of star formation explains the discrepancy between the number of observed Galactic satellites and CDM predictions rather than any modification to the CDM theory such as self-interacting dark matter or a warm dark matter component.
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