My interpretation of this lyric is that Light Yagami is trying to say that it would be a dark world without Light, a.k.a himself, the "God" of the "New World" as he proclaims. In one of the episodes Misa exclaims "I couldn't imagine living in a world without Light!" referring to Light Yagami. To which L replies "Yes, it would be quite dark." so That is really what this lyric is getting at. Here is the link for the video.
https://www.youtube.com/watch?v=zvqtomU_JHg