Adam said
Lightspeed changes in different media due to the absorption and re-emission of photons passing through that medium's electron cloud structure. The electric charge density is what affects c.

I agree with the assumption that light speed changes due to absorption and re-emission of photons.
But is this the only way that photons can advance, in other words, if we see photons that have travelled millions of light years in space, are these the same photons that left that galaxy millions of years ago or have they been absorbed and re-emitted by some “electric charge density” that might exist in the space that the photons have gone through

Since photons exist in space, then space cannot be empty.

Cheers
Johannes