What is the cause of gravitional force of attraction? Ok,this is a silly question. But for quite long I have been thinking about this. What is gravitation all about?? My book directly writes since force of gravitation is directly proportional to the masses and inversly proportional to the square of the distance ie. $$F = G\frac{M_1 . M_2}{r^2}$$ . So is this gravitation all about? What is the cause of it? The book then says

We are not in the scope in classical approach so as to discuss the cause of this. However,as a matter of fact that if there are more than one mass,then there is gravitation as it is their fundamental property.( Cause of gravitation can be attributed to exchange of non-classical particles between the masses) . 

Then it ended the talkings. What are those non-classical particles? Why and how are they exchanged? So far I have studied physics , I never found such weird phenomenon like this. I want to know what is the cause of gravitation and what is the cause of exchange of those so-called non-classical particles? And how do they do it?
 A: Ultimately, the only thing that can explain a physical theory is a better theory. Newton's force law used to be the entire theory of gravitation up until the early 1900's. Newton himself had no explanation for what "caused" the force. 
Einstein gave us General Relativity, which describes how gravity is the geometry of space and time. In GR, energy and momentum curve spacetime, and objects simply follow the straightest possible path through the curved spacetime! So Newton's law is "explained" by GR.
With the success of Quantum Field Theory at describing the other three fundamental forces (electromagnetism, weak nuclear, and strong nuclear), physicists have been trying very hard to create a quantum description of gravity, though none have been completely successful so far. In QFT each force is described by a field. For example, the electromagnetic field. When the field is "excited," or in other words when the field goes to a higher energy state, the excitations of this field are called photons. When you run through the math of QFT, you find that two like charges placed in the photon field will repel each other with a force that varies as $1/r^2$! This is because the two charges raise the energy state of the field. Similarly, what we do know is that if you put two objects in a simplified massless spin-2 field, you get an attractive force between the objects that varies as $1/r^2$! The force comes from each object coupling to the field and lowering its energy. Excitations of this field are called "gravitons." There is as of yet no direct evidence that gravitons exist, and it's highly unlikely that there will be any any time soon given how weakly they are predicted to interact.
A: Currently, there is no generally accepted explanation of why objects attract each other. There are hypotheses such as Le Sage's, but none remained popular. The most basic (simplest) theory of gravitation is still Newton's law of universal gravitation.
