If I were in a vacuum with no gravity or friction, why would it be harder to push a floating bowling ball than a marble? Is there anything happening on an subatomic level that makes matter act differently when it's pushed together? How would matter know to act different unless it somehow interacted with it's environment?
Yes, mass interacts with the environment (space). That interaction manifests as curved space towards center of mass of the body. More mass means more curved space and more inertia - more resistance to change of state.
The curved space (gravity) of the object itself manifests as inertia when you try to change state of the object (e.g. from rest or uniform motion).
Same curved space, manifests as gravity for other objects.
So, if you want to change state of earth, its own gravity (curved space) resists that change in state, giving earth massive inertia.
Same gravity (curved space) causes other objects move towards center of earth.
Gravity and inertia being caused by same (curved space) phenomena demonstrates as equivalence principle.