What is black hole in-universe?
A black hole is a region of space once something enters it can never leave, not even light.
Albert Einstein published his field equations of General Relativity in 1915. Soon after Karl Schwarzschild solved the field equations for the vacuum around a massive body.
The Schwarzschild solution has a singularity where time stops and nothing can escape. It is called the Schwarzschild radius
This means that any mass which gets compressed to a radius below
The Sun is too small to collapse into a black hole, but we can calculate
So, Schwarzschild predicted black holes in 1915. More recently objects have been discovered which are so small and massive that they can only be black holes.