Dark Romanticism is a subcategory of Romanticism, which focuses on emotion, imagination, individualism, intuition, and nature. What makes it Dark Romanticism is that the emotions are dark and unpleasant. Imagination rustles in paranoia or terror. Individualism is from isolation and alienation. Intuition is unreliable and untrustworthy (such as the unreliable narrator in Poe's "The Tell-Tale Heart"). Nature is cruel, tense, and frightening (as seen in Jack London's "To Build a Fire").
In short, Dark Romanticism is a genre of literature that paints mankind as evil and pitiful, while the world is something to be feared.