that's the thing, photons don't have mass. neither do electrons. or at least, not normally. EM drive experiments suggest that there may actually be a way to make electrons into some configuration where they have mass because they were able to bump a pendulum with the beam from one that was in vacuum, where there could not have been mechanical transference force, or any other particle but the electrons emitting from the resonator.

it was the em drive that started me on the idea that gravity is in fact mediated by clouds of these special configurations of electrons. and so they would also distort the velocity of photons as well.

the idea that intergalactic space might mean light speed is orders of magnitude faster than we observe here, throws a monkey wrench into the works of estimating the size of the universe. you CANNOT measure distance without triangulation or firing a projectile into the distance and measuring the time. and even then, even at the ranges that snipers shoot, the bullet path, and velocity, and air resistance is affected so much that at the longer distances it's likely to not even be the time you estimate because you assumed uniformity of conditions over the flight path.

same thing would apply to light, i think, since gravity changes the velocity.