Comment by api

As far as I know it’s the latter and that’s a big problem for physics. A lot of stuff like string theory, loop quantum gravity, etc. require energies that would take a particle accelerator the diameter of the solar system or something nuts like that.

Without tests it’s just pretty math that can be coaxed into agreeing with reality but that proves nothing.

Physicists try to indirectly test all the time via cosmological observations but that is extremely hard and limited to what you can infer and how well you can eliminate other explanations or sources of error.