So what’s so hard about putting a graviton on a scale to see how much it weighs?
“Dark energy” isn’t just making galaxies fly away from us, it is increasing the amount of space between them. For example, the distance from the tip of your nose to the tip of your finger seems pretty stable, right? It isn’t... The atoms, the electrons, protons, neutrons, quarks.... The distance between them (and their constituent parts) is also expanding, but we don’t notice it. Eventually, the space between electrons and their atomic nuclei will be too far for any interactions to take place.