Muhammed- More good questions!
1. I don't know why the curvature of spacetime depends on the mass density. So far as I know, no one knows yet. Perhaps all physical theories will always have a deepest level at which we can only say 'that's how things are'. The level is likely to go deeper, so maybe soon your question will be answerable on the basis of a deeper theory.
2. Gravitons have not yet been observed. It is hard to see how they could not exist, since they are predicted on the bases of General Relativity and Quantum Mechanics. Both these theoretical structures have been found to have very wide domains of validity. They canít be combined in a consistent way that would apply to all ranges of phenomena, but it seems very likely that there are no major tricks in combining them on the scale of, say, milliseconds, a range in which gravitational waves are being sought. Of course finding gravitational waves is not the same as finding gravitons, just as finding light didnít right away mean finding photons.
You may find as you study science more that the line between things that are íprovení to exist and those that are íunproven theoriesí is not completely sharp.
(republished on 07/23/06)