This question is poorly framed. The concept of a photon is of no real use on the cosmological scale, since a photon is a quantum scale phenomenon. It’s not that it isn’t true that a photon emitted at one frequency will, on detection at cosmologically distant location be observed to have a lower frequency. Because E=hf, the energy of a photon is inversely related to its frequency, therefore it is inarguable that the photon has lost energy - because it has a lower frequency than it started out with.

The problem for scientists, who only think in terms of photons, is that it is difficult to explain why the photon lost energy. So you get a lot of dissembling as in Viktor T. Toth's answer to As the universe expands, photons lose energy (redshift). Where does this energy go?

Viktor begins by stating, “Photons do not really lose energy in an expanding universe.” This is tantamount to advising that you not believe your lying eyes. What you are supposed to believe instead, is some tortured mathematicist, mumbo-jumbo. A little relativistic hand-waving and an expanding universe are invoked to explain the cause of the lowered frequency in such a way as to sidestep the question of how to account for the observed energy loss. In other words Viktor’s answer doesn’t answer the question asked; it avoids it.

How can you account for the energy loss? Simple, you have to model light emitted from distant sources on the cosmological scale as consisting of expanding spherical wavefronts. Those wavefronts can be thought of as hyperspheres which are related to the mathematical-geometrical concept of a light cone.

That theoretical concept closely parallels the physical structure of an expanding spherical wavefront considered 4-dimensionally. In the theory of light cones the surface of the light cone is the aggregate of all the hyperspheres which are themselves, sections of the light cone. All of the points on the light cone have no spatial or temporal separation. The 4-dimensional frame of the light cone constitutes a simultaneity - in 4-dimensions.

Deploying this mathematical model in the context of the observed expanding wavefronts suggests that at the point of contact where a 3-dimensional observer interacts with the light cone, at a specific 3-spatial + 1-temporal, dimensional location, the observer is observing the simultaneous state of the entire wavefront, which, in the model, is the state of the hypersphere of the light cone that intersects with the observer’s 3+1-dimensional “now”. The 3D observer cannot observe the entirety of the light cone, only the hypersphere with which it intersects

At large cosmological distances that wavefront will be observed to be redshifted, reflecting a net energy loss to the spherical wavefront. This energy loss is caused by the wavefront’s encounter with and partial absorption by intervening galaxies over the course of its expansion.

This energy loss can be crudely estimated using the standard General Relativity equation for gravitational redshifting. That, in turn, suggests that observed gravitational effects are a consequence of the interaction between 3-dimensional matter and 4-dimensional electromagnetic radiation.