Why gravity only attracts but, doesn't repel? How it came into existence? How it has influence over such large distances? How gravity truly works?

The facts we do know from school.

Sir Isaac Newton laid most of the ground work for 'classical mechanics' . He pictured 'space' as a frame which is holding everything in the universe. He pictured it as real as you and me. Time passes uniformly through the space. So whatever happens within the universe is relative to space itself. So Space and time are absolute.

After he observed an apple fall from a tree, he tried to understand the reason behind it and formulated the famous 'Newton's law of universal gravitation'. We all know about it.

According to his law Every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them.

'F1' or 'F2' - force with which one object is attracting other.

'G' - the gravitational constant( value is 6.674×10-11 N m2 kg-2 )

'r' - distance between the center of their masses

'm1' or 'm2' - masses of both objects

It was the real genius of Newton to figure this out. According to him, every planetary body has this attraction which is called 'gravity'. This is how everything sticks around in the universe. This is how the planets in our solar system orbit around the sun and this is how we can walk and jump on earth.

If we are talking about the gravity of earth or 'g' then the strength of the gravitational field is numerically equal to the acceleration of objects under its influence. 
So if we are considering a body freely falling towards the surface of the earth, then it's acceleration will be equal to the strength of gravitational field of earth. It's value is 9.81 m/s2  or 32.2 ft/s2. This is the number, even every school kid knows. It's being used by us for solving various problems till today. 

Even after Newton The law of universal gravitation was used to predict the locations of planets and natural satellites( secondary bodies that orbit the primary like our moon ) that had not yet been discovered. The eventual discoveries of these celestial bodies confirmed that the law was correct.

So what is the point of all these when we have already know all these  stuffs from school? 

So the point is - how gravitational force is conveyed between objects? How gravity manages to influence objects over such huge distances? What is the physical significance of gravity? Why objects attract each other?

Newtonian gravity couldn't explain the nature of attraction between objects or what drives gravity.

The facts that we might have left out about 'gravity'.

In 20th century Prof Albert Einstein proposed that space and time are warped with each other behaving as a single entity of space-time fabric. The space-time was not rigid anymore, it was flexible. The fabric of space-time can bend, twist and ripple.

Space-time fabric

This new theory opened up a new possibilities towards understanding how gravity really works and to answer the above questions. After working for years Einstein was finally able to crack the secret behind gravity.

The secret was in the nature of the fabric of space-time itself. It was just about the reaction of a bendable fabric like space-time to it's surroundings as it curves in the presence of matter due to it's mass. Just like when we sit on a soft couch,  the couch stretches and deforms by our weight temporarily. When we get up, it again comes back to it's original position. 

So now we know why gravity was the 1st force to come out of the 'big bang' cause gravity is nothing more than the nature of the space-time itself.

Curve in space-time due to mass of an object

Gravity is nothing more than a deformation in the shape of space-time itself. It's the warping in the fabric of space-time due to the mass of an object. The moon revolves around the earth not because of some invisible force but, because it rolls along a curve in the fabric of space-time that the earth creates. This is followed by all the matter in the universe. The objects with more mass creates larger curvatures within which the lighter objects rolls around it.

So space is real. It's not just theory. Space has properties and mass as the matter does.

Ripples in space-time

But it's sometimes confusing to imagine a three dimensional representation with a two dimensional explanation. 

What i mean is if you have already figured out how this new gravity works. Then try to think how this new gravity helps us to stay on the surface of earth. 

If you get it then it's good. But if you haven't then this might be somewhat helpful to you.

As the mass of a celestial bodies is concentrated at it's center so the center point is more responsible for warping space-time.

Now we really have to buy into the concept as mentioned earlier that space-time is real and it is the ultimate frame of the universe which surrounds us. It's kind of invisible to us. The center of mass of an celestial object deforms space-time and creates a bend towards it's center, so small objects like ourselves are constantly being pulled towards the center or it's more like falling towards to the center of the earth as an fruit from a tree, always falls towards the surface of the earth. That is why it's called the 'gravity well'.

The 3D representation of how space-time bends due to an object

We could clearly see how the center of the mass deforms or warps space-time towards itself in the image above.

I hope the explanation was somewhat useful.