TESTS OF GENERAL-RELATIVITY AND NEWTONIAN GRAVITY AT LARGE DISTANCES AND THE DARK MATTER PROBLEM

Observations of the five most simple known 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 its weak field limit, Newtonian gravity, over distances much larger than the size of the solar system. The observations confirm within error bars 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 in clusters of galaxies. Uncertainties and incomplete observational data limit the present statistical accuracy of the tests to about less than or similar to 30%. Future observations may improve significantly this accuracy and reduce the possibilities for systematic errors.