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Black holes keep 'burping up' stars they destroyed years earlier, and astronomers don't know why
(www.livescience.com)
This is a most excellent place for technology news and articles.
It's an oversimplification, but something with the mass of say, a human, will be crushed into a very very very very tiny space. A much smaller space than 0.1mm radius, which would hold Earth's moon's mass of 7.342×10^22 kg.
Yes... And something with the mass of the universe will crush into a significantly larger (yet comparatively tiny) volume of space... Thus, a black hole is basically the maximum density we can currently comprehend and theorize.