A black hole is formed when a mass becomes so large or so concentrated that it generates a bottomless gravitational well from which nothing can escape, not even light. This well has a well-defined edge called a "horizon" . Visually, a black hole therefore looks like a large ball of absolute black. A priori, what happens beyond this limit will remain forever unknown to us since nothing can come out of it. So we have no idea what the nesting object might look like, if there is one. The theory of general relativity predicts that matter is concentrated there in a geometric point of infinite density, which does not really have a physical meaning (in other words, the theory is reaching its limits there).
Read also: The mysterious “cry” of the birth of black holes
We do know what happens when two black holes "collide" . After turning around faster and faster, they end up merging when their respective horizons meet. They then instantly give birth to a black hole
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