Is there solid evidence, not just theorizing, for this? Our bodies can deal with damaged DNA. Proving that low rates of DNA damage increase cancer risk propor

Is there solid evidence, not just theorizing, for this? Our bodies can deal with damaged DNA. Proving that low rates of DNA damage increase cancer risk proportionaly to the risk asociated with high rates of damage seems nearly impossible. So I don't understand the absolute certanty with which it gets proclaimed as fact.