"In the beginning", there was darkness, as in no "light" (Genesis 1:1-3). One needed the combination of light/good and darkness/evil for the creation of matter. Much like you have to have equal and opposite forces, or sour to appreciate sweet. That applies to this "creation". The "new" "heaven and earth"/creation, will not require "darkness", as the "light" will be ever present, with no night. (Revelation 21:23-25)In reality, there isn't really any such thing as "dark", it's just an abstract term we use to refer to there being relatively little or no (visible) light. Physically and psychologically though, we need both "light" and "dark", with too much or too little of either generally not being very good for us.
I think that gives the metaphor in an interesting context.
Revelation 21:23 And the city has no need of the sun or of the moon to shine on it, for the glory of God has illuminated it, and its lamp is the Lamb.