In Newton's view, all objects exert a force that attracts other objects. That universal law of gravitation worked pretty well for predicting the motion of planets as well as objects on Earth and it's still used, for example, when making the calculations for a rocket launch. But Newton's view of gravity didn't work for some things, like Mercury's peculiar orbit around the sun. The orbits of planets shift over time, and Mercury's orbit shifted faster than Newton predicted.
Einstein's idea was that gravity is not a force, but it is really an effect caused by the curvature of space and time. Although all its predictions have almost been confirmed by experiment, General relativity fails to explain details near space time singularities at the centre of Black holes and the mysterious dark matter. Which means Einstein equations cannot explain the motion of stars in galaxies and galaxy clusters. Mordehai Milgrom MOND explains the motion of stars in galaxies correctly without assuming Dark matter. Therefore MOND is an alternative to Newton's law of Universal gravitation. However the most serious problem facing Milgrom's law is that it cannot completely eliminate the need for dark matter in all astrophysical systems.
Most theorists believe that gravitons must exist, and that they could be candidates for Dark matter, because quantum theory has successfully explained every other force of nature. But not everyone agrees. No theory claiming to unify quantum theory with GR has been successfully verified, and this has raised suspicions that perhaps gravity isn't like any other force — in which case gravitons may not exist. The theory of quantum gravity is expected to be able to provide a satisfactory description of the microstructure of space time at the so called Planck scales, at which all fundamental constants of the ingredient theories, c (speed of light), h ( Planck constant) and G ( Newton's constant), come together to form units of mass, length and time. The search for the full theory of quantum gravity has been stymied by the fact that gravity's quantum properties never seem to manifest in actual experience. One option for a solution to this conundrum is string theory, or the idea that everything we perceive as a particle or force is simply an excitation of a closed or open string, vibrating at specific but unique frequencies. One of the major criticisms of string theory has to do not with the theory so much as with theorists. Not only that, the strings of string theory are stupendously small, thought to be somewhere around the Planck scale, a bare 10-34 meters across. That's far, far smaller than anything we can possibly hope to probe even with our most precise instruments. The strings are so small, in fact, that they appear to us to be point-like particles, such as electrons and photons and neutrons. We simply can't ever stare at a string directly.
Therefore emergent gravity or entropic gravity is a theory in modern physics that describes gravity as an entropic force, a force with macro-scale homogeneity but which is subject to quantum level disorder and not a fundamental interaction.
The theory has been controversial within the physics community but has sparked research and experiments to test its validity. The problem is, if emergent gravity just reproduces General Relativity, there's no way to test the idea. What we need instead is a prediction from emergent gravity that deviates from General Relativity.
Finally when all is said and done, the fifth force is proposed and its first result is that it reproduces the MOND and Emergent gravity results from one single force equation and solves all gravitational problems leaving none untouched. Although it is accurate there is one problem; it can't explain the origin of gravity.
Gravity is then fixed by postulating that it is as a result of the Casimir effect due to vacuum polarizations with sounding experimental proof.
