It's really just matter that doesn't emit light.
Oh, it's much funkier than that. It's not just lack of emission, it's the lack of any interaction between light and dark matter. There's no refractive index, no ability to reflect or absorb light, no known way for it to cast a shadow and (to my understanding) there's no effect similar to radiation pressure. You might as well forget about anything relating to (classical) optics when you are talking about dark matter.
I don't know about most physicists being sceptical, but IMO it's a very interesting concept, even if it would turn out that it's about as good as luminiferous aether.