The theory of gravity was developed by Sir Isaac Newton in the 17th century. His groundbreaking work, published in "Mathematical Principles of Natural Philosophy," introduced the concept that every mass attracts every other mass in the universe. Newton formulated the universal law of gravitation, which mathematically describes this attraction based on the masses of the objects and the distance between them. This theory laid the foundation for classical mechanics and significantly advanced our understanding of planetary motion, explaining phenomena such as the orbits of planets and the motion of projectiles on Earth.