Nice timing! A few months ago, I had a thought about two rooms - one in complete darkness and one in complete light.  When the door between the two rooms is opened, the light floods into the darkness, but the darkness doesn't impact the light.

To further the analogy, I was just reading a book that connected some dots for me.  If you have two rooms - one with love and one with evil . . . when the door is opened, the love will flood into the evil room, but (again) not vice-versa.

And therefore darkness and evil are nothing more than words that describe the absence (or varying degrees) of lightness and evil.  Isn't that cool.  I like that perspective a bunch.