Light and dark are archetypal metaphors--universal understood metaphorical constructs. Archetypal metaphors bridge culture and time which is one reason why their usage is so popular and why we talk about Shakespeare's usage of these metaphors. When you think of darkness, what is the first thing that comes to your mind? Trust your gut on this. Fear, uncertainty, depression are all notions that people tend to think of when darkness is present. Now think of light. What comes to mind? Peace, calm, purity? How do these concepts work in Romeo and Juliet? Which character(s) is often described as being in the light (sun) and which character(s) dwells in darkness? Think of the play as a whole. When is it bright and when is it dark? These questions should help you think more analytically for that introduction.