Humans only recently (like in the last 300 years) realized what Gravity is all about.
Greek philosophers thought that the planets and stars were part of the gods' realm and followed a "natural motion." They did not realize that Gravity is involved. The Greeks' ideas stuck around until the 16th century.
Beginning in the 1500s, though, astronomers like Galileo and Brahe discovered that the earth and other planets revolved around the sun. Kepler showed that they moved in an eliptical orbit, not a circle. The question was why.
Sir Isaac Newton -- The Discoverer of Gravity!
Sir Isaac Newton was an English mathematician and mathematician and physicist who lived from 1642-1727.
The legend is that Newton discovered Gravity when he saw a falling apple while thinking about the forces of nature.
Whatever really happened, Newton realized that some force must be acting on falling objects like apples because otherwise they would not start moving from rest.
Newton also realized that the moon would fly off away from Earth in a straight line tangent to its orbit if some force was not causing it to fall toward the Earth. The moon is only a projectile circling around the Earth under the attraction of Gravity.
Newton called this force "gravity" and determined that gravitational forces exist between all objects.
Using the idea of Gravity, Newton was able to explain the astronomical observations of Kepler.
The work of Galileo, Brahe, Kepler, and Newton proved once and for all that the Earth wasn't the center of the solar system. The Earth, along with all other planets,orbits around the sun.
Two astronomers, J.C. Adams and U.J.J. LeVerrier, later used the concept of Gravity to predict that the planet Neptune would be discovered. They realized that there must be another planet exerting a gravitational force on Uranus because Uranus had odd perturbations in its orbit. (Perturbations are deviations in orbits.)
Albert Einstein -- The General Theory of Relativity
Einstein developed a whole new idea about Gravity.
According to Einstein, Gravity arises from the "warping" of space and time.
Einstein's new theory of Gravity explains a number of phenomena that would violate Newton's theory. For example, light bends when passing near massive objects like the Sun. And a clock raised above the Earth speeds up relative to a clock on the surface.