How is darkness a metaphor?

Darkness is the defining metaphor of the modern age. This ascension began in the late 19th century, but with the revelations of the abominations of the Nazis in WWII, it rose straight to the top and nothing appropriate has yet come along to topple darkness from its throne:

“Sometimes I think that if I keep walking, I will find some light, but I know that I can walk to the other side of the world and there will still be darkness. It’s not like the darkness of the night, which also has white light from the stars, from the moon. This darkness is inside me and has nothing to do with the outside world.”