One very simple way to answer this question is that the American Physical Society publishes journals called Physical Review {A,B,C,D,E}. Of these, Phys Rev D is the one focused on high-energy physics.

If there was no other interesting physics requiring further research, how could those other fields be generating enough papers to fill 4 journals?

And this isn't even counting more recent journals like Phys. Rev. Fluids.