Remember the definition of "invisible" -- it means only that light (wavelengths our eyes can detect) does not interact with it. The term "darK" is perhaps more descriptive than "invisible", as it connotes something we have no knowledge of. Yet, that is. Our mathematical models may lead us to a way to understand what's going on. I think the answer will surprise us, when we learn it.
Remember the definition of "invisible" -- it means only that light (wavelengths our eyes can detect) does not interact with it. The term "darK" is perhaps more descriptive than "invisible", as it connotes something we have no knowledge of. Yet, that is. Our mathematical models may lead us to a way to understand what's going on. I think the answer will surprise us, when we learn it.