Mankind knows gravity only by it’s perceived behavior. They do not know what it is exactly, as it is a subatomic particle too small to be seen or studied. However, in recent years they have made a discovery in which they found gravity in it’s natural non-interfered state and they called it “Dark Matter”. Dark Matter and Gravity are essentially the same thing.
How it works is these sub atomic particles travel in a straight line in it’s natural state, and they travel in every direction, but when they come in contact with atomic matter, they are magnetized like our planets and push against the atomic matter in the same way, as they travel through the field of atoms, they begin slowing more and more as density increases. Thus the more mass and density the object the slower the gravity particles can move through it.
When a scientist sends a man to space, they say the subject is now in zero gravity, but the truth is, the person is suspended in space because they are in (absolute gravity), there is nothing around them slowing the particles down in one direction or the other. Now imagine these particles going in every direction through space and these particles find their way through a large planet like earth, the inhabitance of that planet will be subject to those subatomic particles as well. The particles traveling from the right are equal to the particles traveling from the left, and balance is kept. The particles traveling from above pushing you down are much stronger then the particles traveling up coming through the earth, thus we have gravity interference and are held firm to the surface.
Take the moon as an example, we see the effects of this gravity interference with it as well in the tides, when the moon is overhead it slows down the particles which push making the pull just a bit more pronounced causing a swelling of the ocean. It is not possible for two large objects like the moon and the earth to collide due to magnetics and other energies not yet understood. All large celestial objects have a push vs. pull value which cause an orbit, if an object is too small it will not hold this value and fall back to the planet. Gravity can only push something to the earth if it is small in comparison.
There is much interest in the science community to figure out how to manipulate gravity. Though man could potentially figure out a way to accomplish this on their own, it has been held back artificially, (intervened with) as man is not to leave the earth at this time, there will be a time for it in the next century, but they are not ready yet. It would be like giving the car keys to a 5 year old which is how we are viewed in the galactic community.
If man were to travel to the very center of the earth it has been theorized by man that they would be suspended in zero gravity, this is not true, there is never a point where the gravity particles will ever be at absolute anything within the earth, as so many factors are at play, there will always be a push or a pull in one direction or another, but it would be much more balanced then on the surface, thus if you could go to the center of the earth you would feel very light but you would not be suspended.