Darkness is a studied imagination of apocalypse in which the world has become a "lump of death" and darkness is itself the universe.
Byron takes delight in heaping up the details of horror. To some extent it represents the underside of romanticism: the triumph of its opposite, the end of all the enlightenment held dear.
Humanity and freedom are defeated; faith is parodied in the image of one dog's pointless loyalty to the corpse of its master; God is replaced by the Fiend as Famine; desolation and mockery rule.
It is, however, "not all a dream" for it conceives, dramatically, philosophically, of the end of meaning and value.