The no-hair theorem postulates that all black hole solutions of the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum. All other information (for which "hair" is a metaphor) about the matter which formed a black hole or is falling into it, "disappears" behind the black-hole event horizon and is therefore permanently inaccessible to external observers. Physicist John Archibald Wheeler expressed this idea with the phrase "black holes have no hair" which was the origin of the name.
There is still no rigorous mathematical proof of the no-hair theorem, and mathematicians refer to it as the no-hair conjecture.
Read more about No-hair Theorem: Example, Changing The Reference Frame, Four-dimensional Space-time, Extensions, Counterexamples, Black Holes in Quantum Gravity
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“To insure the adoration of a theorem for any length of time, faith is not enough, a police force is needed as well.”
—Albert Camus (1913–1960)