Many gravitationally lensed quasars exhibit flux ratio anomalies that cannot be explained under the hypothesis that the lensing potential is smooth on scales smaller than one kpc. Micro-lensing by stars is a natural source of granularity in the lens potential. The character of the expected fluctuations due to micro-lensing depends sensitively on the relative surface densities of micro-lenses (stars) and smoothly distributed (dark) matter. Observations of flux ratios may therefore be used to infer the ratio of stellar to dark matter along the line of sight -- typically at impact parameters 1.5 times the half light radius. Several recently discovered systems have anomalies that would seem to be explained by micro-lensing only by demanding that 70-90% of the matter along the line of sight be smoothly distributed.
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