"In physics, there is no broad consensus as to an exact definition of matter. Physicists generally do not use the word when precision is needed, prefering instead to speak of the more clearly defined concepts of mass, energy and particles."
http://en.wikipedia.org/wiki/Matter

Energy and mass are really the same, as energy is mass multiplied by the squared speed of light in vacuum. Particles are relatively durable relative unities, which means they are not really particles (absolutely durable absolute unities) at all. However, where Newtonian physics worked with the concept of particles, quantum physics works with the concept of quanta:

"In physics, a quantum (plural: quanta) refers to an indivisible and perhaps elementary entity. For instance, a "light quantum", being a unit of light (that is, a photon)."
http://en.wikipedia.org/wiki/Quantum

This is the concept of the particle once again, as a quantum presupposes a definite quantum. Like the particle, the quantum is, to speak with Nietzsche, yet another example of "the soul superstition".

This belief, from which derives our idea of "substance", is the idea of the subject. It is the idea that "I" am something absolutely durable and indivisible, separate from my body (which, after all, is neither durable nor indivisible). Grammar - our Indo-European grammar, at any rate -, being founded on this belief, tends to strengthen this belief. But it is still a belief, and not knowledge.