We study the amplitude of the weak gravitational lensing signal as a function of stellar mass around a sample of relatively isolated galaxies .
This selection of lenses simplifies the interpretation of the observations , which consist of data from the Red-sequence Cluster Survey and the Sloan Digital Sky Survey .
We find that the amplitude of the lensing signal as a function of stellar mass is well described by a power law with a best fit slope \alpha = 0.74 \pm 0.08 .
This result is inconsistent with Modified Newtonian Dynamics , which predicts \alpha = 0.5 ( we find \alpha > 0.5 with 99.7 % confidence ) .
As a related test , we determine the MOND mass-to-light ratio as a function of luminosity .
Our results require dark matter for the most luminous galaxies ( L \mathrel { \raise 1.29 pt \hbox { $ > $ \kern - 7.5 pt \lower 4.3 pt \hbox { $ \sim$ } } } 10 ^ { 11 } L% _ { \odot } ) .
We rule out an extended halo of gas or active neutrinos as a way of reconciling our findings with MOND .
Although we focus on a single alternative gravity model , we note that our results provide an important test for any alternative theory of gravity .