It starts with a simple idea: there is no difference between a
force due to
acceleration and a
force due to
gravity.
Albert Einstein showed that this is the case if
spacetime (he had already shown that
space and
time were really a common entity in his theory of
special relativity) is
curved, and massive objects curve spacetime in their vicinity.
Pretty soon, you have black holes, wormholes, and all manner of weirdness.