Electromagnetism, both nuclear forces, and even the Higgs force are mediated by known bosons. What about gravity? Does it require gravitons?
If you examine the Universe extremely closely, by probing the fundamental entities with it on the smallest possible scales, you’ll discover that reality is fundamentally quantum in nature. Matter itself is made up of indivisible, uncuttable quantum entities: particles like quarks, leptons, and bosons. These quanta have charges (color charge, electric charge, weak isospin and weak hypercharge, and “mass/energy” as a gravitational charge), and it’s the exchange of quanta between these charged particles (gluons, photons, W-and-Z bosons, etc.) that mediate these forces. There’s even a Higgs force as well.
However, one type of quantum that’s never been detected is the graviton: the theoretical particle that mediates the gravitational force. Even though it’s predicted to exist (and to have a spin of 2, unique among all particles) and, just like light is composed of photons, gravitational waves should be composed of gravitons, those predictions rely on an unproven assumption: that gravity is fundamentally a quantum force in nature. Is that assumption necessarily true? It isn’t in Einstein’s General Relativity, and that prompted…
