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 because 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 when it moves. 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 while experiments confirm that space contracts and time dilates, Einstein’s equations give no clue as to how that happens.
Particles collide when they move into the same space at the same time, but time and space aren’t like that. A particle can’t collide with time or space because it exists within them, so how can it curve space and dilate time when it moves, as Einstein says? A billiard ball can’t change the size of the table it rolls on, nor can it slow down a movie of it doing so, but our space and time do contract and dilate when we move, so how can that happen? Relativity then challenges the basic concepts of a particle model by allowing particles to change their space and time.