What causes motion?

Answer 1

If anything is acting on a partilce, it will keep moving along a straight line, with constant velocity (in all the inertial reference frames). In this sense, nothing causes the motion.

A change in this kind of motion, that is, an acceleration or a curve in the trajectory, is caused by a force, according to the Newton equation F = dp/dt. "Nowadays" we prefer to avoid the concept of force F, using instead the idea of field, which is independet from the particle and from its parameters (mass, charge, ... ): you can say that the field permeate the space, whether the particle is there or not.