Einstein believed quantum mechanics was incomplete, as per him if two particles were entangled, they must carry a hidden variables which is pre-determined instruction sets that tell them how to behave when measured.
Coming to 1964, John Bell devised a mathematical way to test this. He showed that if Einstein were right (if local hidden variables existed), there would be a strict limit on how correlated the measurements of two distant particles could be. This limit is known as Bell’s inequality.
Quantum mechanics predicted that entangled particles would be more strongly correlated than any local theory could allow, thus violating Bell’s inequality.
Starting with John Clauser in 1972 and followed by Alain Aspect in 1982, experiments have consistently shown that nature violates Bell’s inequality.
This experimental confirmation led to the 2022 Nobel Prize in Physics for Alain Aspect, John Clauser, and Anton Zeilinger.