I've always questioned this. I once read that light will also have a red shift as it passes through intergalactic dust and gas. So all galaxies will show a red shift, and the farther away they are, the more dust and gas the light will pass through. Hence a larger red shift.
But I don't have anything like a degree in science. I'm just a dumb lawyer, so I wouldn't bet the farm on my "theory."
“I’ve always questioned this. I once read that light will also have a red shift as it passes through intergalactic dust and gas.”
As I understand it, that is not Doppler shift but rather the absorption and re-emission of light at a lower frequency, such as absorbing light, which warms the gas/dust particles, which re-emit the energy as heat.
Not to worry!
Distant galaxies are, at this very moment, beginning a Class Action lawsuit against Einstein, Hubble and Planck.
Higgs, of boson fame, is their attorney.... or was that Higgins of Magnum, PI fame?
The “red shift” that scientists observe isn’t just a general redening, it is a shift in a characteristic frequency of certain atoms, typically hydrogen. The issue of gas and dust affects the intensity of light, which has play in observations of distant “standard candle” supernovae.