Question: what is a black hole and why is it formed?

Roan Haggar answered on 9 Nov 2020:

Black holes are very mysterious, and there’s still a lot we don’t know about them! They are very, very dense objects, weighing between thousands and trillions of times as much as the Earth, but with all that mass squashed into a tiny space.
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Because they’re so heavy and so dense, black holes have a very strong gravitational pull. In fact, the gravity of a black hole is so strong that not even light can escape from it, despite light being the fastest thing in the Universe! This is what gives them their black colour, as none of the light that goes into a black hole can come out again.
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Different sized black holes form in different ways. ‘Stellar-mass black holes’ form when a very large star (much bigger than our Sun) runs out of gas to burn. With nothing else to burn, the gravitational pull of the star will make the it collapse, forming a dense black hole. These are the type of black holes that have been found by gravitational wave detectors in the last few years.
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There are also much larger black holes, called ‘supermassive black holes’. Scientists are a bit more unsure about how these form, but there are several ideas, ranging from smaller black holes merging together, to black holes pulling lots of stuff into them to grow over time. Most galaxies (including our galaxy, the Milky Way) have a supermassive black hole at their centre. You may have seen the photo of a black hole that came out last year — this was a picture of the supermassive black hole at the centre of a galaxy over 50 million light-years away!
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https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole
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I hope this helps!