The majority of the atom, any atom, even an atom of the most solid material, is empty space. Atoms are intrinsic units due to the tiny electrons orbiting the the nucleus at light speed that bind it together and lead to atoms forming together and repelling each other through molecular chemistry.
These electromagnetic and gravitational energy forces, part of which Rob described in greater detail with the "virtual particles" are present in the large
virtually empty space of the atom,
but it is without matter or mass as we think of the terms. There's a nucleus and then there's lots of empty space, and then there's the tiny electrons buzzing around at light speed.
Now, true, scientists are trying to work out gravity as both a
particle and a wave (like light) in their theoretical study of the gravitron nanoparticle, but essentially, the space is empty. I'm not saying that Rob is wrong, and I am not wrong either. It's just that there are points to be elucidated further. It's interesing stuff since obviously the gravitational force, which occurs at the speed of light, is what orders the natural laws of the universe.
http://education.jlab.org/qa/atomicstructure_10.html
If atoms are 99.999999999999% empty space then why don't things pass right through them?
Things don't fall through other things because they are levitating on an electrostatic field! I am not kidding! When you sit on a chair, you are not really touching it. You see, every atom is surrounded by a shell of electrons. This electron cloud presents a rather negative face to the world. Remember that like charges repel each other. When two atoms approach each other, their electron shells push back at each other, despite the fact that each atom's net charge is 0. This is a very useful feature of nature. It makes our lives a lot easier.
Now the question you should be asking is, if atoms push away from each other, why doesn't the entire universe just blow away from itself? The answer is that some, actually most atoms' electron shells are not full. When two atoms come together and have empty spaces in their electron shells, they will share electrons to fill in the spaces in both of their shells. Yes, the electrons really do go back and forth between atoms and they do so pretty fast. Electrons tend to be kind of mobile, which is also a very nice feature of nature, since without it your walkman would not work. Once both atoms' outer shells are full due to this electron sharing, they go back to their usual repulsive behavior. This, by the way, is how we get molecules and the secret to understanding Chemistry. It's all about the electrons!