What proof do we have that the graviton actually exists? Quote from http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html" "The graviton is the exchange particle for the gravity force. Although it has not been directly observed, a number of its properties can be implied from the nature of the force. Since gravity is an inverse square force of apparently infinite range, it can be implied that the rest mass of the graviton is zero. " How do we know so much about the graviton when we have never actually seen or even detected one?