We know that gravity is a very weak force compared to electromagnetic forces and the nuclear forces. We know about the other forces because they're necessary to explain atoms, and we can detect gravity easily, because unlike the other forces it is always attractive, so the weak gravitational force from every particle in a planet can add up to a measurable effect.
However, is it possible that there are other fundamental forces that are much weaker than gravity, or even of a similar magnitude but with "charges" that attract their opposite and therefore cancel out over large scales? It seems that we wouldn't necessarily have detected such a force if it did exist, and it seems different from the kind of thing you can probe using a particle accelerator.
I realise this question is kind of naïve. I know a little quantum mechanics but never studied quantum field theory or particle physics. I'm curious about whether those formalisms provide a way to rule out such additional weak forces.