I'm trying to study the best approaches to quantum gravity and especially the interactions of quantum and the metric. But first let us settle about the so called "spacetime points". What is the proof that spacetime points can't be composed of any substance but purely an abstract. The often arguments are that general covariance and diffeomorphism invariance require the spacetime points are not composed of any substance or something that can be tracked in time. I want to know all the proof of this from actual experiments. Is there any or all just a theoretical reasoning based on beauty and symmetry with no actual experimental justifications?