There are several major groups and also many independent researchers who reject the conclusions reached by mainstream mathematical physicists.
Make no mistake. These dissidents have no rational theories to offer in exchange. The main reason is that opponents of the establishment propose nothing different than those whom they attack. They either explain their theories with discrete particles or are in the business of moving concepts.
The main issue a dissident has to address is the structure of the atom. There is yet to be found a dissident who does not use the planetary model of the atom or some other discrete model. And the only issue a dissident has to resolve in this context is to identify the entity that maintains the integrity of the atom. What entity or medium or thing prevents the electron bead from flying away? And how does one atom affect another at a distance if there is no mediator in between?