system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know the actual wavelength of light originating from the cluster of galaxies that are receding away from us when all we observe is red shifted light because of expansion?
gpt
Every element has its own characteristic spectrum of light, so we can look for this fingerprint in the light we receive. Since hydrogen is by far the most abundant element, we expect the spectrum of hydrogen to feature prominently. However, the features of the hydrogen spectrum won't appear at the same wavelengths as they do when we measure hydrogen in the lab, because of the red shift. 

Instead, the spectrum is shifted by a certain amount. But despite the shift, the structure remains unchanged, so we can identify the spectral lines of hydrogen in the light we collect. And by the amount it has shifted from what we measure in the lab, we can obtain an estimate for the speed with which the source is moving away from us.