According to: https://en.wikipedia.org/wiki/Cosmic_microwave_background the CMB (Cosmic Microwave Background) "is faint cosmic background radiation filling all space"
Also, https://en.wikipedia.org/wiki/Vacuum#Outer_space says "no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter"
And https://physics.stackexchange.com/tags/vacuum/info: "This rather theoretical requirement is never achieved in practice, because even if space does not contain any atoms / electrons / nucleons, it does contain a lot of photons and neutrinos. But we still call it a vacuum, as an approximation of the theoretical vacuum."
Then on https://en.wikipedia.org/wiki/Speed_of_light we have "The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second (approximately 300,000 km/s (186,000 mi/s)). It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second"
Given that, at least according to Wikipedia, in practice vacuum does not exist, and it's filled with radiation, is it actually possible to measure the speed of light in vacuum? Also, has anyone even observed light in vacuum, ever?
If not, what types of vacuums has light been measured in? Are there any records about this?
Additionally, if true vacuum doesn't even seem to exist and it's filled with radiation (CMB), can we really assume that light doesn't need a medium to propagate? Wouldn't it effectively be propagating through whatever is filling up the space? Would we ever be able to tell the difference?