Intergalactic dark matter A relatively recent research brings evidence for the presence of the dark matter not only inside and near the galaxies, but also in intergalactic space: Matter Distribution around Galaxies, S. Masaki, M. Fukugita and N. Yoshida, Astrophys. J. $746$, (2012) 6.
Could this intergalactic dark matter alter initial frequency of the photons coming from very distant galaxies?
 A: The answer can only be affirmative if the intergalactic dark matter interacts weakly with the photons received on the Earth after a very long travel through intergalactic space. Or, we have sound arguments in this regards, as long as the weak interaction between electrons and dark matter is already acknowledged, and electron-positron annihilations or creations of electron-positron pairs from photons prove beyond doubt the total identity between the matter of electrons and photons.
More, as the mass $m_γ$ and the frequency $ν_γ$ of the photons are directly proportional, $m_γ = hν_γ/c^2$, we can calculate the accurate ratio between the smallest mass $m_{γ(min)}$ of a photon and the electron rest mass $m_0$ by using the minimum frequency of a photon $ν_{γ(min)} = 4.08\cdot10^8 Hz$ found by Penzias and Wilson as the lower limit frequency for universal background of microwaves, and the frequency $ν_a = 1.234\cdot10^{20} Hz$ of the photons resulted from electron-positron annihilation at rest, $$m_{γ(min)}/m_0 = ν_{γ(min)}/ν_a = 3.306\cdot10^{-12}.$$ As seen, this value corresponds exactly to both the axion mass (now estimated to be of order $10^{−11}$ times the electron mass) and the non-dimensional constant of the weak interaction (on the scale where the non-dimensional constant of the strong interaction is equal to $1$). 
As known, the axion was hypothesized in 1977 by Peccei and Quinn to solve a paradox arising from how the strong nuclear force affects antimatter and matter, more concretely it can explain an unexpected symmetry whereby that force has the same effect on matter as it does on antimatter. Axions must be stable, have a very low mass and cross section, and couple weakly both between them and to all elementary particles, including photons and neutrinos. And later many and many researchers began to believe that these very light axions could be the main or even the unique components of the dark matter, just because these subquantum particles can solve at once several essential problems related to the dark matter, such as galaxy ration curve, bullet cluster, CMB, large scale structure formation, etc. All these arguments have been so convincing that  in the past decades many nuclear and particle physicists began experiments to detect decidedly the particle dark matter in the event that it interacts as such with the normal matter, although right from the start the success of such attempts is doubtful due to the infinitesimal size the light axions certainly have.
Moreover, such extremely small fundamental subparticles were already predicted by Georgi, who wrote in 1981 as a conclusion of his analysis on all the known fundamental interactions “At distances of the order of $10^{-29}cm$ the world can be very simple and contains only one kind of particles” (Sc. Amer. $244$ (4), 1981, 40). And this conclusion is well correlated with the effective section of about $10^{-47} m^2$ experimentally measured for weak interactions, which means an action radius of about $10^{-24} m$ for weak forces, which in turn means an axion dimension much below its action radius. Also, this exceedingly small size of the axions explains how two elementary particles, for example two electrons, interact weakly between them: “Sometimes they even say the weak forces act only at a point. Thus, the collision of two particles interacting weakly resembles the collisions of billiard balls.” (D. B. Cline, А. K. Mann and C. Rubbia, Sc. Amer. $231$ (6), (1974) 108).
Therefore, the above calculated data confirm not only the reality of axions as “universal matter” (Heisenberg, Physics & Philosophy, 1959) or as “a kind of a general basic material” (Mukhin, Experimental Nuclear Physics, 1974) out of which all elementary particles are made as “different forms in which matter can appear”, but also the unique nature of weak and strong fundamental interactions, whose very different strengths derive just from the very different numbers of axions acting simultaneously between them in a certain interaction, weak or strong (for a correct appreciation of the last assertion one must remind synthesis of hadrons from high-energy longitudinal electrons and positrons coming from opposite directions, $e^- + e^+ → hadrons$).
The weak internal cohesion of the photons is another essential point for deciphering the photon-dark matter interaction. Differently from the indestructible electron, whose physical entirety is never affected by its collisions with other elementary particles, however violent they are, the photon splitting is well known, for example in Compton or Raman scattering, which prove that an electron can reflect or absorb only a part of the incident photon, and the rest becomes a new photon with correspondingly smaller mass and frequency, or the absorption of a single photon by two atoms in touch, or the splitting of one photon in two or several fragments which then can interfere between them, etc. This much weaker internal cohesion of the photons proves that their subquantum components are much weaker bound between them, in comparison with those existing inside the electrons, and consequently in the intergalactic spaces these very less bound subcomponents can individually be eliminated from the photons as a result of their very rare (but not entirely inexistent) collisions with the particle dark matter, and thus a very long travel of the photons in intergalactic space results in a progressive decreasing of their mass $m_γ$ and frequency $ν_γ = m_{γ}c^{2}/h$, decreasing evidently directly proportional to the covered distance.
In conclusion, the presence of the particle dark matter in intergalactic space oblige us to take into account a new corpuscular mechanism able to justify the Hubble redshift.
