Hints: Hubble Space Telescope telltale M87

To convince the skeptics, they needed to find more supermassive black holes in many more galaxies. For this they needed to look further into space. So they turned to Hubble Space Telescope. From 1994, Hubble began a systematic survey of the centers of distant galaxies, searching for the telltale signature of stars speeding around a supermassive black hole.
Astronomers started by looking at an active galaxy, M87. As expected, it had a giant feeding black hole shooting a great jet of energy into space. But it was when the search broadened out to include inactive galaxies as well that something incredible happened.

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