Gravity is a primary force of nature, exerting attraction on objects in relation to distance. A 'gravity field' is the area of gravity attraction. Gravity is generally expressed as a force which is applied according to relative mass in space, where a larger object has more mass, and therefore more gravity than a smaller object. This force is believed to be the cause of black holes, galactic vortices, and the movement of stars around a galactic centre, either defined, as in spiral galaxies, or undefined, as in galactic 'clouds'. Another form of gravity is specific gravity, which is best explained as the relative gravity of matter in solution, reacting to the forces acting on mass in the solution.