Definition: Matter has many definitions, but the most common is that it is any substance which has mass and occupies space. All physical objects are composed of matter, in the form of atoms, which are in turn composed of protons, neutrons, and electrons.
Photons have no mass, so they are an example of something in physics is not comprised of matter. They are also not considered "objects" in the traditional sense, as they cannot exist in a stationary state.
