I just said that in th Big Bang Teory they are not, and my question was what is the mechanism of avoiding being the subjest to friction.

Any moving object in the universe that has some gravitational mass (which is the same as inertial mass, and therefore relativistic mass, and so non zero for photons) is a subject to loss of energy because of its gravitational interaction with the surroundings. It is called "dynamical friction" (as I learned from Ned Wright). So it is a universal thing.

This gravitational interaction necessarily causes a loss of energy (on average, excluding "sling effects"). That's why the Big Bang Theory needs a mechanism to compensate for this loss of energy, in other words, mechanism of creation of this tiny energy that would be seen as redshift if it were not compensated for. That's why I'm asking about the mechanism of creation of this energy lost to gravitational interaction.

I'm not an astronomer to do the observations and interpret them. I just know how to calculate some stuff and I can see that if there wouldn't be such mechanism you (the astronomers) wouldn't see the expansion. Since those two numbers (Hubble redshift and dynamical friction) seems very close to each other, maybe identical. So I'm asking what mechanism compensates for the dynamical friction of photons making it non existent? Eg. John Baez suggests it is exchanging of momentum for energy. But for me it is too vague to understand. That's why I'd like someone who understands the mechanism to explain it.