Tests of General Relativity at Large Distances and Dark Matter -
  The most simple observed cases of gravitational lensing of distant quasars and galaxies by galaxies and clusters of galaxies are used to test Einstein's theory of General Relativity and Newtonian Gravity over galactic and intergalactic distances. They extend the distance range over which Newton-Einstein Gravity has been tested by 10 orders of magnitude. Although the precision of the tests are far from the precision of the solar system tests of EGR and those from pulsar timing in close binary systems, they confirm quite accurately the validity of Einstein's General Relativity and its weak field limit, Newtonian Gravity, over galactic and intergalactic distances and the existence of large quantities of dark matter in galaxies and clusters of galaxies. Future observations can improve the accuracy of the tests and reduce the possibilities for systematic errors. 
Arnon Dar