Gravitation is the force of attraction between two masses. It is one of the four fundamental forces of nature, along with electromagnetism, strong nuclear force and weak nuclear force. Gravitation affects objects on a macroscopic scale, from stars and galaxies to planets and moons. On a microscopic scale, it holds atoms together in molecules and binds particles in nuclei. Gravitational fields are created by any mass or energy that has non-zero mass-energy; they cause objects to accelerate towards each other with a force proportional to their masses. The strength of this force decreases with distance according to an inverse-square law. In addition, gravitation is responsible for the formation of large structures such as galaxies and clusters of galaxies, as well as the warping of space-time around black holes.