It binds stars in clusters and holds clusters of galaxies together. It even controls the expansion of the universe. The answer is the conservation of energy and momentum and in this case, specifically angular momentum. It is angular momentum that prevents the planets from falling into the Sun, confining them all to orbit in the same plane. It is angular momentum that creates and maintains the disk shapes of galaxies and prevents clusters of stars and galaxies collapsing down to a central point.
Conservation laws are the most fundamental laws in physics.
The most familiar example of the conservation of angular momentum is the ballet dancer, whose spin increases as she draws in her arms. Similarly a planet orbiting the Sun, or a galaxy orbiting the common centre of gravity of a cluster of galaxies, has a certain angular momentum and an energy which is the sum of its gravitational potential energy and its kinetic energy. The energy and momentum must both be conserved. The closer a planet's orbit is to the Sun the lower its total energy. The only way the planet or galaxy could fall into the centre of the system is by losing more energy.