In the book, characters often wonder “what lies beyond” and “what is beyond the bounds of the universe.” And the reader realizes how little he or she knows of the universe. This theme is mostly relevant towards the ending of the book. So for me this is just a side-theme. The main theme is about revenge and how much Roland ( the gunslinger) wants to catch the man in black. Other side-themes could be: survival (because of the constant struggle for the gunslingers survival), and afterlife (because of the many conversations on the topic, for example: when the gunslinger asks Brown if he believes in an afterlife, and he answers: “I think this is it”.) That line is one of my favorites from the book.