I have explained this several times. The speed of light constant, is observed by all observers has unvaried, not measured at a constant , but seen at a constant, unvaried, 3 dimensional space is invisible to all observers, empty, light is not seen in this space. If we removed all matter, accept one star, we would still be able to see in the constant.Right - that's how vision works.
What is the 'unvaried constant of the invisible?' (Have a feeling I will regret asking that question, but what the heck.)