Well that's just a theory, albeit good, and there's a good chunk of modern science that refuses to marry it, so let's not be too sure about it, shall we?

Edit: Wikipedia summarises it nicely:

Although general relativity is highly regarded for its elegance and accuracy it is not without limitations: the gravitational singularities inside of black holes, the ad hoc postulation of dark matter, as well as dark energy and its relation to the cosmological constant are among the current unsolved mysteries regarding gravity; all of which signal the collapse of the general theory of relativity at different scales and highlight the need for a gravitational theory that goes into the quantum realm.

Edit: Also highly recommend this read: https://plato.stanford.edu/entries/quantum-gravity/