Why do many cultures associate "white" with good and "dark" with bad?

I’m an Indian living in Norway with my family. I have an 11-year-old son, and recently he asked me something I couldn’t answer clearly. While walking on snow covered streets, he noticed that snow is described as “clean” or “good” when it’s white, but when things spoil or decay they’re described as turning brown or black. He asked why “bad” things so often become dark, while “good” things stay white. He wasn’t talking about people, but it made me realize how widespread these associations are - across languages, metaphors, and everyday descriptions. I’m curious how others think about this: Is this mainly rooted in physical processes (decay, mold, light)? Is it linguistic convention? Cultural history? Or something else entirely? I’m not looking for political arguments - just trying to understand how these symbolic associations developed and how to explain them honestly to a child. Would appreciate thoughtful perspectives or references.