Disregarding expansion of space, are there any processes which can redshift light over billions of light-years?

Assume there are 10 molecules per cubic meter, on average, in interstellar space--in effect, a very thin gas. Assume that the source of light is ten billion light years away, the tunnel through which it passes is one meter square, and space does not expand. If there are $$9.46052840500002\times10^{25}$$ meters in a direct path 10 billion light years long, the light will potentially interact with $$9.46052840500002\times10^{26}$$ molecules in the path.

Can gravitational lensing, scattering, propagation delay in photon-electron interaction, or any other process along the path cause light from the original source to arrive at the detector with lower frequency? In other words, will it be red-shifted?

• Light passing near a moving black hole (or other massive, compact object) could be redshifted or blueshifted. See this question. – Peter Shor Oct 12 '18 at 12:48
• If you're trying to find an alternate explanation than the expansion of the universe for the redshift from distant objects, I am sure that any explanations that use only standard, known physics have already been considered and ruled out. – Peter Shor Oct 12 '18 at 12:51

It is a usual practice in physics not to use more digits than required for a given problem. Since what you are making is an estimate, just an order of magnitude would suffice, say $$10^{26}$$ metres.