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. In the closed model, the universe will recollapse to zero size some 100 - 200 billion years form now. In the open models the universe expands and cools forever. In both cases, spacetime expands from an initial point called the Big Bang in which the size of the model is arbitrarily small, and the density and temperature of the matter soars to infinity. Still it is a highly curious feature of science to predicate of the universe a 'beginning', an event which is the cause of future events, but not the effect of previous causes.