I think (a) could work, but in (b), if matter were absorbing light for an infinite span of time, it would eventually heat up and glow itself. “Dark” matter does

I think (a) could work, but in (b), if matter were absorbing light for an infinite span of time, it would eventually heat up and glow itself. “Dark” matter does not directly interact with electromagnetism aka light, so wouldn’t block it.