To: Mak10
You want a simple answer to a reletively simple question, yes? Well, here you go.
Step one: Scientists don't know. This is why they speak of things such as "gravitons" (some unknown object that does some unknown act to produce a well-known result).
Step two: I'm not sure either. I've got a theory (a simple one that fits everything I think), but I don't really know. Keep searching and find experiments to test your theories. This will be the next step in understanding the physics involved in the world around us.
Now we begin. My personal theory involves what I like to call "energy particles" (much similar to light waves -- I think the only difference would be frequency). Assuming my theory to be correct, there are millions upon billions (rough guestimate) of these particles traveling through space at intense speeds. Presumably, they travel at light-speed. They are smaller than atoms (probably much smaller), and numerous to fill the emensity of space (for all intents and purposes). These energy particles collide with atoms constantly. Due to the weak force each particle exerts on the objects and the overwhelming abundance of them, net force can be taken to be 0 for each object. When two objects are put near each other, they shield each other from some of the energy particles. this causes a change in the net force. Thus Gravity.
I can't prove it, but I can't disprove it either. And it makes sense. Also, if you'd like proof of the exsistance of these "energy particles" check out "crystal radios." Go ahead and build one yourself; get it working, etc. Now, turn from your favorite station. Pass the country stations. That's right, keep going... Stop! There it is: static. Not enough, eh? Take out the speaker, and plug in a voltometer. Now you've got it: enough energy to light a tiny bulb. Not enough for practical use, but it's there. Not a broadcast; it just exsists. I'm not sure where it comes from, but it's out there.
Hope it helps. Oh, and remember...
"look for what is... not what you think should be." -direct quote from an inquisitive mind.
enjoy.