If god doesn’t exist, How do you explain rainbows?

No I’m not combating against Christianity or supporting atheism i just really want to know if god doesn’t exist, who’s the mother fucker that made up an entire story about the world flooding just to explain why rainbows exist? Who the fuck would do something like that? It’s things like this that make me hope god exists because if he doesn’t then the Bible is just the biggest fanfiction ever made and I don’t think I could handle people dying for decades over a fanfiction. In that sense it’s easier to believe in a lie and have just a little more hope for humanity.