Gravity was defined by Newton. It is the name for the acceleration between any object that has mass and all other objects that have mass.
There are a lot of names for what must be between two massive objects in order for them to attract each other. It has been called space-time, the ether, gravitational force, and other things. Some people seem to think that there is really nothing in between the objects, but then those people need to resort to a mystical forth dimensional force to explain gravity, of course, there is no real answer when you need to explain how there can be both a "mystical force" and "nothing" between the two objects at the same time.
Let's face it, there really must be something between two objects in order for one object to affect the other one. I really like Einstein's name for it the best: "space-time".
There have been many experiments done that will allow space- time to be better defined. The best one is what happens when a particle of mass is accelerated to a speed that approaches the propagation speed in the space-time. When this is done, the particle begins to take on mass.
Where does the mass come from ? according to Einstein, the energy is converted into mass. Great - now we need to define energy before we can further define space- time. Ok, let's keep it simple. Energy is what causes mass to move. That is all of the definition that is needed. Energy can be converted into a motion or a compression of mass.
If causing a mass to move (near light speed) makes the mass become more massive, then whatever the mass is moving through must also be mass.
The particle must be made of the same stuff as what it is moving through, or you will really get lost in trying to explain it. E=MC2 pretty much says this, and I think that Einstein knew, but didn't write it down.
One single type of stuff makes up both the particle and what it is traveling through (weather the stuff is composed of particles or one substance is a good question).
Ok, now we have this stuff called space-time that looks like ordinary mass when it is compressed. The particle - wave duality is solved because, in order for a compressed zone of mass to be stable, it must be in constant motion.
Einstein's famous equation E=MC2 is satisfied for atoms because the
center mass of an atom accelerates to near light speed before bouncing
back on itself, thus sustaining the constant motion required to
a zone of space-time that is more dense than the ambient space time.
of motion is maintained as a stable oscillation of the mass. The
speed is in Einstein's formula because that determines where the
occurs (and this determines how much energy a certain amount of mass
to maintain a stable oscillation). The sphere expands and contracts
a period of oscillation that
depends on the amount of mass in the sphere and the density of the ambient medium.
When the sphere is expanding and the rate of expansion approaches light speed, the sphere bounces back toward it's center. A shock wave, or shell is also created. This shell consists of the ambient medium that was displaced by the expanding sphere.
Shells are formed that are at different energy levels for each atom (because of the oscillation frequency and the diameter). These different densities determine what frequencies of light will be absorbed by the atom - the density of the crest of the light must be close to the density of the outer shell of the atom in order to be absorbed. Photons will be emitted when the center mass of the atom is not able to sustain the density of the outer shell. Personally, I like this allot better than trying to count electrons that you are not supposed to know the location and speed at the same time.
The atom allows something else to be added to our definition of space-time. Time can be defined as the relative motion of objects. We choose a certain type of object (a cesium atom) and count the oscillations of the atom. The problem is that the frequency of the atom depends on the ambient density of the surrounding space-time. This has been verified by experiment. If you have two atomic clocks, one on earth's surface and the other in synchronous orbit, the clocks will run at different speeds. This was put forth by Einstein. As the density of space-time increases, time slows down because the amount of energy that is required for a certain frequency of motion is greater. If more energy is required to move at the same rate and there is no extra energy applied, the rate will slow down. If a photon of a certain energy moves from a higher density to a lower density zone, the speed of the light will increase and the wavelength will also increase. In Einstein's words "light will shift towards the red as it leaves a massive object".
So now we have enough information about the space-time to be useful. Any mass will produce a higher density in the surrounding space-time as it looses energy, and all of the density gradients will combine to produce acceleration of the objects towards each other. Particles travel through the space-time as oscillating spherical compression waves, and some types of particles travel as a motion that is a sustained high density zone (like the wake of a boat sometimes produces a hump that stays a hump as long as the speed is maintained). Matter begins as a compressed zone and anti-matter begins as a rarefied zone.
Unlike other theories, this one has no elaborate "proof", but it describes a method of constructing a computer model of a single substance by describing the properties of each voxel (small area in space) and how the voxels inter-relate. It is a premise of this theory that by modeling this single substance, all of the complex atoms and particles may be viewed by inserting a stimulus (a compressed zone of the same substance that is being modeled). As the model is allowed to progress into time, the compressed zone(s) will behave similar to real world photons, electrons, atoms and all of the other particles and "fields" that are observable within our present four dimensions.