That's a pretty slippery topic. I think the big brains and high-power cosmologists can spend a month discussing it and not have an answer.
For US ... people who are not big brains or high-power cosmologists ... I think the best way to think about it is that MASS causes inertia.
The more mass an object has, the more inertia it has. And all of the math and Physics that describe the behavior of mass can also be understood as the results of inertia.
