Short answers, pulled from the story.
John Michell published a letter in 1784 suggesting that stars could exist so massive that their gravity would prevent even light from escaping. Pierre-Simon Laplace independently proposed this idea in 1796. Michell calculated that a star with the same density as the Sun but 500 times its radius would have an escape velocity exceeding the speed of light.
The first direct image of a black hole and its vicinity was published on the 10th of April 2019. This image followed observations made by the Event Horizon Telescope in 2017 of the supermassive black hole in Messier 87's galactic centre. The image showed a bright ring of light surrounding a dark shadow consistent with predictions from general relativity.
The Schwarzschild radius is the boundary where gravity becomes so strong that nothing can escape. Karl Schwarzschild found the first exact solution to Einstein's equations revealing this radius in 1917. This radius marked the boundary where gravity becomes so strong that nothing can escape.
The LIGO Scientific Collaboration and Virgo Collaboration announced the first direct detection of gravitational waves on the 11th of February 2016. This event named GW150914 represented the first observation of a black hole merger. The black holes were approximately 1.4 billion light-years away from Earth and had masses of 30 and 35 solar masses.
Stephen Hawking predicted that black holes emit small amounts of thermal radiation in 1974. This effect known as Hawking radiation arises from applying quantum field theory to black holes. The temperature of this radiation is of the order of billionths of a kelvin for stellar black holes making it essentially impossible to observe directly.