A quantity E G probing general relativity (GR) on large scales (a hundred billion times the size of the solar system) for the first time has been measured with data from the Sloan Digital Sky Survey to be E G = 0.392 ± ± 0.065 (~16%) consistent with GR, GR plus Lambda CDM and the extended form of GR known as f ( R ) theory, but ruling out a particular TeVeS model predicting E G = 0.22 . This estimate should improve to ~1% with the next generation of sky surveys and may put tighter constraints on the parameter space of all modified gravity theories.