Disclaimer: I’m not a particle physicist, but I am a theoretical physicist. My info on this subject might be outdated. My understanding is that the “electrowea

Disclaimer: I’m not a particle physicist, but I am a theoretical physicist. My info on this subject might be outdated. My understanding is that the “electroweak unification” (electromagnetism + weak force) is considered experimentally confirmed, whereas the “grand unification” (electroweak + strong force) is something most physicists believe to be possible but the theories remain somewhat speculative. For instance, if true, such unification likely means that the proton should be an unstable particle that very slowly decays, and that has never been observed experimentally. Unifying such a “grand unified theory” with gravity (general relativity) is however a much harder problem: While all the other forces are already understood in terms of quantum mechanics, gravity is currently understood in terms of geometry, and it’s not obvious how to combine those in a sane way. That’s where string theory, quantum loop gravity, etc, come in; but AFAIK, most prospective theories are very far from even experimental predictions, let alone experimental testing, and not everyone even agrees on the premise that gravity “should” be described using quantum field theory. We just know that our current theories can’t be complete, since thought experiments like “what is gravity during a quantum double slit experiment” or “what really happens near and inside a black hole” can’t be answered in a fully satisfactory way using current theories.