Things would need to have 'mass' in order to have inertia and be defined somewhere in space-time? If 'charges' did not have mass where would they exist (relative to each other)? More mass = more inertia (harder to change position). Order the position of charges with mass to allow them to interact. You can't have one giant planet for some reason. You need lots. If you had one giant planet 'where' would it exist?