Why does milton describe the world as dark
If you notice the link I have provided below you will see one of the many references to the point made in your question.
Milton, being a strict Calvinist, looked at the world in a very structured way. Calvinists see in everything the direct intervention of God himself. Whether good or bad, events transpire in life in a way that reflect an irreversible and unchangeable plan formulated by God from eternity.
As he outlines the obvious applications of "light" and "darkness" in scripture it seems as though he is following along with the common practice of picturing "good" as light and "bad" as darkness, and there is an undeniable element of that involved.
But almost every scholar that discusses this question agrees that when Milton refers to "this dark world" he is making reference to his blindness which came about as a result of the side effects of the glaucoma from which he suffered.