Main Question or Discussion Point
Speed of light through vacuum is 299 792 458 m / s, but is slower through a denser medium like glass or water. We now know vacuum is not empty as previously thought, but has all kinds of new material in them like dark matter. Has any experiments been done to determine how fast light can be thorough a medium that has no dark matter in it? Would it be intuitively faster than the currently measured speed of light through vacuum?