How did scientists know about the existance of black holes, how they behave etc… long before getting the very first image of one
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There are two things going on here.
First, before the first image of a black hole (produced in 2019), we had other experimental evidence for them. Thanks to studying the centre of our galaxy over long time periods, we can see that stars are orbiting something very massive but non-luminous: [https://www.space.com/41291-relativity-revealed-milky-way-core.html](https://www.space.com/41291-relativity-revealed-milky-way-core.html). Also, experiments to detect gravitational waves were successful, and detected signatures matching predictions of black holes spiralling into and colliding with one another: [https://www.scientificamerican.com/article/gravitational-waves-discovered-from-colliding-black-holes1/](https://www.scientificamerican.com/article/gravitational-waves-discovered-from-colliding-black-holes1/).
Second, before either of these things, nobody “knew”. They did, however, have interesting predictions made by playing around with the equations of General Relativity. GR had been experimentally verified to high precision – for example, GPS works (when launched, the satellites had the option to enable GR corrections or not, and they had to enable the corrections else the positions drifted rapidly thanks to the effects of the satellites moving fast and being in weaker gravity than on the earth’s surface). But if you try and work out what happens with a very dense blob of matter, you realise that if you get close enough to it that not even light can escape its gravity. Such an object is a totally black hole in space… hence the name. Scientists then did more calculations and predictions, ran models of galaxy formation, etc etc, and guided the above searches to look for them.
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