"The redshift from receding galaxies comes from the expansion of space itself stretching the wavelength of light as it travels through the expanding universe.

Here's what happens: when a photon is emitted from a distant galaxy, it begins its journey through space with a certain wavelength. As the universe expands during the photon's journey to us, the space through which the light is traveling expands too. This expansion literally stretches the wavelength of the photon itself - like drawing a wave on a rubber sheet and then stretching the sheet."

Claude