Detection of weak gravitational lensing distortions of distant galaxies bycosmic dark matter at large scales

Most of the matter in the Universe is not luminous, and can be observed onl y through its gravitational influence on the appearance of luminous matter. Weak gravitational lensing is a technique that uses the distortions of the images of distant galaxies as a tracer of dark matter: such distortions ar e induced as the light passes through large-scale distributions of dark mat ter in the foreground. The patterns of the induced distortions reflect the density of mass along the line of sight and its distribution, and the resul ting 'cosmic shear' can be used to distinguish between alternative cosmolog ies. But previous attempts to measure this effect have been inconclusive. H ere we report the detection of cosmic shear on angular scales of up to half a degree using 145,000 galaxies and along three separate lines of sight. W e find that the dark matter is distributed in a manner consistent with eith er an open universe, or a flat universe that is dominated by a cosmological constant. Our results are inconsistent with the standard cold-dark-matter model.