[DiogenesKynikos]

> For a simplified example, the lowest density of interstellar space is 100 molecules per m3. The number of water molecules in water is 3.3E28. If a photon travel 3.5E10 light years (35Bly)

Galaxies are nowhere near 35 billion light years across. Large galaxies are a few tens of thousands of light years across. Once you get outside galaxies, density drops by further orders of magnitude. In other words, your "simplified example" is utter nonsense.

What's surprising to me is that you assume that physicists are complete ignoramuses who haven't even bothered to do the simplest calculations. Do you really think that no one has ever sat down and calculated the effect of foreground absorption on the CMB? There are entire PhD theses on this one subject.