There are 2 accepted theories which describe, unfortunately at classical level, the gravitational interaction:
*Newton’s theory does not answer the question “why?”, but the questions “how?” and “how much?”.
*Albert Einstein’s theory answers every possible question (except the ones regarding behavior at quantum level) including “why?”.
He states that gravity is determined by (present whenever there is) a nontrivial (nonzero) spacetime curvature (for empty spacetime (no matter) the field equations for the gravitational field are:Riccie tensor=0).He asserts that in a flat spacetime (as the ones encountered in special relativity and quantum field theory) gravity is absent.
The logics is:curved spacetime->gravity and the other way around gravity->curved spacetime.