Let's face it, there really must be something between two objects in order for one object to affect the other one. There have been many experiments done that will allow space- time to be better defined. The best one is what happens when a particle of mass is accelerated to a speed that approaches the propagation speed in the space-time. When this is done, the particle begins to take on mass. Ok, let's keep it simple.
Energy is what causes mass to move.
That is all of the definition that is needed. Energy can be converted into a motion or a compression of mass. The particle must be made of the same stuff as what it is moving through, or you will really get lost in trying to explain it. MC2 pretty much says this, and I think that Einstein knew, but didn't write it down. Ok, now we have this stuff called space-time that looks like ordinary mass when it is compressed.