How is it that dark matter can neither emit nor absorb light?
The inability to interact electromagnetically (which is the same as the inability to absorb or emit light) is apparently a fundamental property of dark matter. All of the particles we know about that make up known matter interact via one or more of the known forces. There are four known forces: gravity, the electromagnetic force, the strong force, and the weak force. The fundamental nature of a particle determines the forces it interacts with. For instance, quarks interact via all four forces. Electrons interact via gravity, the electromagnetic force, and the weak force (but not the strong force). Neutrinos interact only via gravity and the weak force. Dark matter particles (whatever they are) interact via gravity, but we don’t know whether they interact via the weak force or not. We know they do not interact electromagnetically, or else we would be able to see them.
