The force that keeps us all glued to the surface of Earth, gravity dominates any discussion of the evolution and fate of the universe. Surprisingly, for all of its impact, it ranks as the weakest of the four fundamental forces in nature (the others being the electromagnetic and strong and weak nuclear forces). But the others pale when you talk about the universe as a whole because the two nuclear forces act only over very short distances, while most large objects are electrically neutral and therefore unaffected by the electromagnetic force.
Isaac Newton first described gravity and had the insight to realize that the force that holds us to Earth (and makes apples fall) is the same one that keeps the planets in their orbits around the Sun. He deduced the mathematical nature of the mutual force and correctly hypothesized that gravity acts across the entire universe. Albert Einstein modified this view of gravity by arguing that the gravitational force is a manifestation of the curvature of space-time. Although Einstein’s idea is necessary for describing the evolution of the universe as a whole, Newton’s theory works well enough when gravitational forces are not extremely strong