Okay imagine a photon is travelling over a very large distance in space, and happens to not hit anything, like all the light from the stars and galaxies we can see from Earth.
now this light exists as a wave between two points, like this
A/’’\../’’\..B
The wave is not hit or modified by any matter or forces on the way that would affect its energy/wavelength/“colour”. other photons from the same source may be interfered with, decreasing the intensity of that source, when viewed from Earth, but we’re talking about the photon that “makes it through”
Now here’s the funny bit. Space itself is expanding. So, over large distances (×) the length of the wave itself becomes stretched (along with the space it is occupying along the way) so the wave exists between the same two points, but the distance between them has increased. Now we cant just get more waves of light from nowhere (to fill the gap), so the original number of waves is stretched out, like this
A/’’’’\..../’’’’\....B
So, due to the expansion of space, light that travels very long distances is appreciably modified, its wavelength becoming longer. This is called “redshift” because longer wavelengths are towards the red end of the spectrum. So a galaxy which is getting further away from you (all of them, due the expansion of space) looks a bit more red than it really is.
Now if you think about it, speed of light being constant, a longer wavelength means fewer waves hitting the target per unit time (lower frequency). Considering the fact that the waves are bearing the energy, it is clear that redshifted light will be of lower energy than in its original state.
This is a principle known as “Doppler Shift” and is exactly the same as that, which explains the modification of tones of sound, as objects move towards or away from the observer.
e.g. The formula 1 car sounds more high pitched, as it approaches, because the sound waves are squashed into higher frequencies, and lower as it departs, because the sound waves are stretched out into lower frequencies.
So, although the actual effect is due to increase in distances, i.e. the expansion of space, it is impossible to travel in space, without being subject to it, so I’d say this is an effect of “distance travelled”
See Hubble Constant and @flutherother‘s redshift link for more detail.