In lecture notes that I have about black holes and space-time metrics it says:
The Schwarzschild solution, which is appropriate outside a spherical, non-spinning mass distribution, was discovered in 1916. It was not until 1963 that a solution corresponding to spinning black holes was discovered by New Zealander Roy Kerr. This solution leads to the possible existence of a family of rotating, deformed black holes that are called Kerr black holes.
Angular momentum is complicated: In Newtonian gravity rotation produces centrifugal effects but does not influence the gravitational field directly. But angular momentum implies rotational energy, so in general relativity rotation of a gravitational field is itself a source for the field.
I understand that in Newtonian mechanics rotation invokes a centrifugal force, how to reconciliation this in the case of gravity is something I hope somebody could elaborate here. thank you!