A theory in which the four space–time dimensions of the universe that are apparent make up a surface, called the brane, in a higher-dimensional space–time, called the bulk. One of the appealing features of the brane world is that it can explain why gravity is much weaker than the other forces, with the nongravitational forces being localized to the brane and the gravitational force not limited to the brane. See also ekpyrotic universe; Randall–Sundrum scenario. A considerable amount of work has been done in using the brane world picture to understand elementary particles.