Gravitational lensing by dark matter in the galaxy cluster CI 0024+17, Hubble Space Telescope image. The cluster galaxies are bright yellow, the far more distant lensed galaxies appear as elongated blue smears. Gravitational lensing is an effect whereby the path of light from distant objects is bent and magnified by the gravity of a sufficiently massive foreground object in the same line of sight. The amount of lensing can be used to determine the mass of the foreground object. By this method, the cluster was found to contain a vast ring of dark matter (see image R980/230), providing the best evidence yet of its existence. Image taken by the Advanced Camera for Surveys on the HST in November 2004.