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This rich galaxy cluster, known as Cl 0024+17, is allowing astronomers to probe the distribution of dark matter in space. The presence of dark matter can only be detected indirectly. Blue streaks near the centre of the image are smeared images of very distant galaxies, not part of the cluster. They appear distorted because their light is bent and magnified by the powerful gravity of Cl 0024+17, an effect called gravitational lensing.


CREDIT: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University).