Black holes are formed of matter so densely compressed together than they attract anything that gets close. Even light cannot escape from it, so they are difficult to see.
A black hole can form after the death of a star, some also were formed at the beginning of the universe!
Black holes are a huge amount of matter packed into a very small area. This pressure forms a gravitational field that is so strong, nothing can escape from it.
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time46gap commented on :
NASA maybe an article about it here —> https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html