Expansion does not occur in a gravity well. So right here in the Milky Way Galaxy, we do not observe space expanding. The effect is simply overwhelmed by gravity.
But in the areas between galaxies, space expands.
As these areas expand, they "add" against each other the further outward from our point of view that you go.
We can make pretty precise measurements of Magnetism and have many working theories and models about it. But as to what spacetime is? For now, it must be a mystery that we are intent to solve.
A simple example is taking the potential energy in a piece of wood and setting the wood on fire to change the form of the energy into heat. Or... bending the stick of wood to transform that potential energy into the form of kinetic energy.
Magnetism is a form of fundamental energy in the universe. These may be the basis of energy itself, since electricity, Electromagnetic radiation, the strong and weak nuclear forces and magnetism are all interrelated.
Is it possible that it transfers through an under-layer of spacetime that we cannot see? Sure, certainly. But we do not know that currently. As it is, things like virtual particles popping in and out of existence are a bit of a mystery. Along with some others discussed recently. For all our iPods, it feels like we are living in an uninformed age, at times.
--Inter Arma Enim Silent Leges--
“Science needs the light of free expression to flourish. It depends on the fearless questioning of authority, and the open exchange of ideas.” ― Neil deGrasse Tyson
"When photons interact with electrons, they are interacting with the charge around a "bare" mass, and thus the interaction is electromagnetic, hence light. This light slows the photon down." - BuleriaChk