Gravity is one of the four fundamental interactions/forces of Nature, and is certainly the first interaction/force that people studied over centuries, dating back to Aristotle (4th century BC), to Galileo (late 16th century and early 17th century), to Johannes Kepler (mid 17th century), to Isaac Newton (late 17th century), and to Albert Einstein (1915).

The law of gravity is the Einstein equations, which was derived by Albert Einstein based on two fundamental principles: 1) the principle of equivalence (PE), and 2) the principle of general relativity (PGE). The PE basically says that the gravitational interaction is the space-time curvature effect. The PGE is a symmetry principle, and amounts to saying that the law of gravity should be indepdent of (covariant) of the way we see it (coordinates).

Remarkably, PGE, together with the simplicity of laws of Nature, uniquely determines the energy functional for gravity, called the Einstein-Hilbert functional (action). The Enistein equations are then the variational equations of the Einstein-Hilbert  functional. 

Due to the presence of the dark matter and dark energy phenomena, the Einstein equations need to be modified. For this purpose, Dr Tian Ma and I (2012) derived a new law of gravity based on the Einstein's two principles: 1) the principle of equivalence (PE), and 2) the principle of general relativity (PGE), and on energy-momentum conserved variation:  

With the new law, we show that the dark matter and dark energy are intrinsic properties of gravity. 

References

• Tian Ma & Shouhong Wang, Gravitational Field Equations and Theory of Dark Matter and Dark Energy, Discrete and Continuous Dynamical Systems, Ser. A, 34:2(2014), 335-366; see also arXiv:1206.5078.
  
• Tian Ma & Shouhong Wang,  Mathematical Principles of Theoretical Physics, Science Press, 524 pages, 2015 

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