Usually when people say something is untestable they don't mean like the Higgs Boson. It was testable, they just needed the proper tools. This was always known and was being actively worked towards. An untestable theory is one that cannot be tested regardless of the tools due to the nature of the theory. Multiverse is an example of an untestable theory. There is no way we know of to actually prove or disprove it. And its fair to question their legitimacy as theories. As testing a hypothesis is the core action of the scientific method, it logically follows that anything untestable is fundamentally not science and therefore not a theory.