Einstein's General Theory of Relativity posits that gravity is not a force but a curvature of spacetime caused by mass and energy. This curvature affects the path of everything that travels through it, including light, leading to phenomena like gravitational lensing. The theory also predicts that massive objects warp space-time around them. Furthermore, the theory's equations, when applied to the universe as a whole, naturally suggest an expanding or contracting universe, a prediction later supported by observational evidence like Hubble's law.