If matter is a substance, objects collide because two substances can’t occupy the same space at the same time. Waves superpose but substances don’t, so objects collide. Newton therefore assumed that matter only moves when another matter particle hits it.
Yet gravity ignores this convention, as the sun holds the earth in orbit from millions of miles away with only space between, so how does it do that? Following Newton, the standard model proposes that graviton particles cause gravity, but Einstein’s cause wasn’t a particle at all.
He proposed that matter can change space and time. Special relativity lets matter alter its own time and space when it moves, and general relativity lets a large object change the time and space of the matter around it. More details follow, but experiments confirm that space contracts and time dilates as Einstein predicted, although his equations give no clue as to how that happens.
Relativity denies that matter is a substance because a substance can’t change what defines its existence. Can a painting change the space that holds it, or a film change the speed of its projector? Yet space does contract and time does dilate, so the space-time canvas of our universe that objects are said to be painted on change.