It's my understanding that materialism is the position that all things are matter. I.e. atoms, molecules, quarks, etc.
But I've often heard people including other things in this philosophy, which I don't understand.
Space-time, forces, or even a massless photon. How are these things matter, therefore material?
My interpretation is that a concept like space-time is an immaterial concept, induced using rationalism, to explain reality.