Often dense, dark and brooding, Ellen Gallagher’s paintings, collages and films encompass a poignant exploration of the African-American experience as well as a rather personal cosmology surrounding nature. While perhaps a bit unsatisfying as a first U.S. museum survey, “Don’t Axe Me” nevertheless contains a number of significant high points spanning two decades of the New York-based artist’s work. Among those is a recent series of lush and luminous paintings of abstracted sea birds and aquatic creatures.