Albert Einstein did not directly discover black holes, but his theory of general relativity, published in 1915, provided the mathematical framework that predicted their existence. General relativity is a theory of gravity that describes the behavior of massive objects and the curvature of spacetime.
In his theory, Einstein proposed that gravity is not a force acting at a distance, as described by Isaac Newton's theory of gravity, but rather the result of the curvature of spacetime caused by massive objects. He suggested that the presence of mass and energy could curve spacetime, causing objects to follow curved paths.
In 1916, just a year after publishing his theory, Karl Schwarzschild, a German physicist and astronomer, found a solution to Einstein's equations that described a region of extreme curvature in spacetime. This solution, now known as the Schwarzschild metric, described a singularity—a point of infinite curvature—and an event horizon beyond which nothing can escape, not even light. This mathematical solution was the first indication of what would later be called a black hole.
However, at the time, the concept of a black hole was not well understood or widely accepted. It took several decades for physicists and astronomers to explore and develop a better understanding of black holes based on Einstein's theory of general relativity.
In the mid-20th century, the work of scientists like John Michell, Subrahmanyan Chandrasekhar, and Roger Penrose furthered our understanding of black holes. Their research provided additional insights into the formation, properties, and possible observational signatures of black holes.
It is worth noting that Einstein himself initially had doubts about the physical reality of black holes. In fact, he and physicist Nathan Rosen published a paper in 1935 suggesting that black holes may not actually exist. However, subsequent research and observations over the years, including the discovery of pulsars and the detection of gravitational waves, have provided substantial evidence for the existence of black holes as predicted by Einstein's theory of general relativity.