How do scientists prove that light "lives" billions of years?

[Attiyah Zahdeh]

How do scientists prove that light "lives" billions of years?

[gyro]

Observational redshifts from distant galaxies/clusters.
Photons are stable particles....no decay.

[KirkDamborg]

thanks for the explination gyro.... i was going to ask you more but i suppose i will save you the time and just try to google the terms in your answer lol ;)