In 1915, Einstein published the general theory of relativity. This is a theory of gravity which is consistent with special relativity . In it, matter and spacetime are interrelated physical phenomena.
Matter moves along curved paths due to the curvature of spacetime.
At the same time, spacetime is curved because of the presence and distribution of matter. Hence the theory is highly non-linear and interactive. The simplest solutions to general relativity are for a single, point mass and for a uniform distribution of point masses. Astronomers still debate the existence of black holes, but evidence mounts in their support. Another simple solution leads to Big Bang cosmology in its most basic form, where the point masses in the theory represent clusters of galaxies.