Gravitational lensing is a powerful tool for the study of thedistribution of dark matter in the Universe. The cold-dark-matter modelof the formation of large-scale structures (that is, clusters ofgalaxies and even larger assemblies) predicts the existence of quasarsgravitationally lensed by concentrations of dark matter so massive thatthe quasar images would be split by over 7arcsec. Numerous searches forlarge-separation lensed quasars have, however, been unsuccessful. All ofthe roughly 70 lensed quasars known, including the first lensed quasardiscovered, have smaller separations that can be explained in terms ofgalaxy-scale concentrations of baryonic matter. Although gravitationallylensed galaxies with large separations are known, quasars are moreuseful cosmological probes because of the simplicity of the resultinglens systems. Here we report the discovery of a lensed quasar, SDSSJ1004 + 4112, which has a maximum separation between the components of14.62arcsec. Such a large separation means that the lensing object mustbe dominated by dark matter. Our results are fully consistent withtheoretical expectations based on the cold-dark-matter model.