Heard about this guy from a PBS Spacetime video, he predicted the existence of black holes back in 1783, found him overall pretty fascinating. From the wikipedia article:

In a paper for the Philosophical Transactions of the Royal Society of London, read on 27 November 1783, Michell was the first to propose the existence of black holes, which he called "dark stars".[15] Having accepted Newton's corpuscular theory of light, which posited that light consists of minuscule particles, he reasoned that such particles, when emanated by a star, would be slowed down by its gravitational pull, and that it might therefore be possible to determine the star's mass based on the reduction in speed. This insight led in turn to the recognition that a star's gravitational pull might be so strong that the escape velocity would exceed the speed of light. Michell calculated that this would be the case with a star more than 500 times the size of the Sun. Since light would not be able to escape such a star, it would be invisible. In his own words:

If there should really exist in nature any bodies, whose density is not less than that of the sun, and whose diameters are more than 500 times the diameter of the sun, since their light could not arrive at us; or if there should exist any other bodies of a somewhat smaller size, which are not naturally luminous; of the existence of bodies under either of these circumstances, we could have no information from sight; yet, if any other luminous bodies should happen to revolve about them we might still perhaps from the motions of these revolving bodies infer the existence of the central ones with some degree of probability, as this might afford a clue to some of the apparent irregularities of the revolving bodies, which would not be easily explicable on any other hypothesis; but as the consequences of such a supposition are very obvious, and the consideration of them somewhat beside my present purpose, I shall not prosecute them any further.

— John Michell, 1784[16]
Michell suggested that there might be many "dark stars" in the universe, and today astronomers believe that black holes do indeed exist at the centers of most galaxies.[3] Similarly, Michell proposed that astronomers could detect "dark stars" by looking for star systems which behaved gravitationally like two stars, but where only one star could be seen. Michell argued that this would show the presence of a "dark star". It was an extraordinarily accurate prediction. All of the dozen candidate stellar black holes in our galaxy (the Milky Way) are in X-ray compact binary systems.[17]

Michell's ideas about gravity and light interested William Herschel, who tried to test them with his powerful telescopes.[15] A few years after Michell came up with the concept of black holes, the French mathematician Pierre-Simon Laplace suggested essentially the same idea in his 1796 book, Exposition du Système du Monde.[3]

It has been written that Michell was so far ahead of his time in regard to black holes that the idea "made little impression" on his contemporaries.[3][11] "He died in quiet obscurity", states the American Physical Society, "and his notion of a 'dark star' was forgotten until his writings re-surfaced in the 1970s."[3]