I'm wondering if the list of particles in the SM is both complete and unique? Or could we find other particles and interactions that could also be included in the SM? Phyisicists sometimes invent fields and particles to account for Dark matter and inflation, etc. But that introduces particles not in the SM. And so my question as to whether the SM is complete and unique. Would any new particle imply a new symmetry besides the U(1)SU(2)SU(3)?Thanks.