All matter experiences the force of gravity. It is the weakest of all the forces of nature, but it is the dominant force in astronomy and cosmology. It controls the movement of planets in our solar system, the structure of stars, the shapes of galaxies and the ultimate fate of our universe.
The concept of a universal law of gravity was first presented by Isaac Newton in the 17th century and it can explain the properties of most astronomical systems. However, when bodies are moving at high speeds or in strong gravitational fields, Newton's theory is inadequate and we need Einstein's theories of relativity. Einstein's special and general theories of relativity play an essential role in understanding black holes, active galaxies and quasars.
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