Cluster of galaxies Cl0024+1654, with visible galaxies in red and the inferred distribution of dark matter in blue. Jean-Paul Kneib and collaborators used the Hubble Space Telescope to map more than 7000 galaxies in the background of this cluster. The shapes of these background galaxies are slightly distorted by the gravitational field of the mass in the cluster, a phenomenon known as "weak gravitational lensing." A computer model was used to determine the distribution of dark matter required to produce the observed distortions. From the European Space Agency.

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