A Hubble image of galaxy cluster MACS J0717, which contains a huge amount of information about the cluster itself thanks to the light from background galaxies. Image credit: ESA/Hubble, NASA and H. Ebeling.

Galaxy clusters prove dark matter’s existence

Clayton Christensen

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Ethan Siegel

Ethan Siegel

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The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.