If matter is a substance, objects collide because two substances can’t occupy the same space at the same time. Waves might superpose, but substances don’t, so objects collide. Newton therefore assumed that matter only moves when other matter 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 assumed that gravitons cause gravity, but Einstein’s cause wasn’t a particle at all.
He proposed that matter changes space and time to make objects move. Special relativity lets matter alter its own time and space, and general relativity lets a large object change the time and space of objects around it. More details follow, but experiments confirm that space contracts and time dilates as Einstein’s equations describe, but they give no clue as to how that happens.
Relativity challenges the particle model because a substance can’t change the time and space it exists within. A painting can’t change the space that holds it, nor can a film change how fast a projector runs it, but space does contract and time does dilate, so what canvas can allow that?