We've only measured gravitational waves directly last year. The theoretical graviton is still ways off - the energies required are incomprehensible to mere mortals.
the graviton is too small to be observed directly by our machinery.
if you can provide a detector the size of the solar system, proving/disproving its existence will be simple.
we could derive anything from first principles if we knew them. theres nothing probable about that. thats the whole point of first principles. do you want me to call you captain obvious?