The dark matter hypothesis may not actually be falsifiable, however that is not what physicists are working on. Physicists work on dark matter models, precisely defined theories which incorporate dark matter and these are falsifiable. For example roughly ten years ago models of weakly interacting particles (that is, particles that interact via the weak force) were very popular, but they have a influence on the abundance on nucleons in the early universe. Turning the argument around, one gets a interaction probability of the particles, and that part of the parameter space was excluded a few years ago. (You can of course try to mask the signal, but then you are generating different models, the original very simple models were falsified.)