Symbolism, Imagery, Allegory
Paradise Lost is about the loss of…well, Paradise. So it's no surprise that images of paradises abound. First and foremost, we have the Garden of Eden. Milton makes it abundantly clear in Book 4 (our first view of paradise) that this is the best paradise of them all. He mentions a number of famous artistic and literary paradises, only to say that the Garden of Eden is much better than them all. In addition to Adam and Eve's Paradise, there's Heaven. It's really bright there, and it doesn't even really get dark.
Now, you might be saying to yourself, well Adam and Eve lose Paradise too. This is true, but there are two other paradises to compensate for this one. First, in Book 12, Michael tells Adam that if he lives the perfect Christian life (he doesn't call it this because Christianity hadn't been invented yet), if he learns to love God by obeying him, he shall "possess/ A paradise within…happier far" (586-587). In other words, paradise is no longer imagined as a place like Hawaii, but rather as an internal sense of peace or calm that occurs when one obeys God as one should. Finally, at the end of time those who have been saved will be able to live either in Heaven or on earth because the earth will have been turned into a paradise, and a much better one than the Garden of Eden. As Michael tells Adam: "for then the earth/ Shall all be paradise, far happier place/ Than this of Eden, and far happier days" (12.463-465).