Dark copse refers to a small group of densely grown trees that obscure light, giving off a gloomy and somber atmosphere. There are several synonyms one can use for the phrase "dark copse." One of them is "thicket," which describes a dense cluster of bushes and trees, making it impossible to see through them. Another synonym can be "grove," which describes a small group of trees planted together. "Shrubbery" is another one as it refers to an area with a dense group of shrubs and bushes. If you're looking for a synonym that's more specific, you could try "woodland," which describes a small wooded area or "coppice," which refers to a cut wood where new growth has sprouted.