what do we know about matter?

'you might as well make a claim like "material objects occupy space and time and have mass."' Energy, I think, rather than mass. It is energy that is the more fundamental concept for physics. Photons have energy. Of course, energy is itself a causal property; that is, it logically entails that particles behave in certain ways. It is not an intrinsic property of the particle; and I don't think we can know what a particle really is apart from its relations. This all belongs to the philosophy of physics, but I would suggest, as a preliminary attempt at elucidation, that matter consists of elementary particles; that an elementary particle is a particular state of affairs with a certain spatio-temporal location, and with certain causal properties and relations to other states of affairs in other spatio-temporal loci; and which is such that in certain formations it is the cause sense-qualia in living creatures. But the nature of these states of affairs is unknown to us beyond their effects on each other and on human perceptual apparatus. So long as we can make a distinction between universals and particulars I think we can frame Physicalism without having to make any ruling on the status of universals; see my earlier post: https://www.boychat.org/oc/messages/157850.htm