c is the constant as it appears in fundamental physical equations, relativistic or quantum. Anything slowing down the propagation of photons through an apparent vacuum (such as interaction with dark matter) which did not affect, for example, the mass-energy equivalence of E=MC2, would not win the bet.
Is that c the speed of light in vacuum or c the constant in special relativity?
c is the constant as it appears in fundamental physical equations, relativistic or quantum. Anything slowing down the propagation of photons through an apparent vacuum (such as interaction with dark matter) which did not affect, for example, the mass-energy equivalence of E=MC2, would not win the bet.