I interpret this as a question about the demarcation of what is science. It is of current controversy in relation to string theory, where the limit on distance scales that can be probed without an accelerated particle creating a blackhole, may put direct phenomena forever beyond our reach. For this reason string theory is dismissed by many experimental physicists as metaphysics. There is even a kind of schism with the main alternative, loop quantum gravity: "Loopy people go to loopy conferences. Stringy people go to stringy conferences. They don't even go to 'physics' conferences anymore." - Jorge Pullin
But there are avenues. Early universe phenomena and their impact on cosmology, blackhole collisions, super high energy cosmic ray collisions, which could provide insight and tests.
What about, areas of our universe which have passed beyond an event horizon? Hubbles law is speed is proportional to distance, so some we will never be able to see.
What do we mean by before the big bang, if space-time was created then?
There are lots of cases like this. Calling them metaphysics is only for comedy effect though. They are informed speculations. And it is a matter of taste among physicists, how far beyond the data they think we should go (and where the funding should be). Popper would call all this part of the process of hypothesis generation, which cannot be driven only by evidence, it has to be creative. It can certainly be part of science.
Because string theory, or m-theory, is going so far beyond experiment, it relies largely on the aesthetic appeal of ideas and rules, like the conservation of information, and symmetry considerations. An example is the octionion-based mathematical structure E8, which seems to have a relation to the number and properties of fundamental particles. But, exists in 8 dimensions, with the set innour universe being a lower dimensional projection like a.slice through. We might also consider various formulations of parallel universes. These kind of explorations seem to blur the boundaries between mathematical discovery, and science. There are patterns and regularities, exploration, and integration like the fairly settled number of dimensions expected now by string theory.
Higher dimensions, parallel universes, the insides of black holes, before the big bang, and outside tge visible universe. These may be firever beyobd experiments. But they can, are, cobtributing to hypotgesis geberation which may givecrise to future laws. All laws and theories are to some extent tentative. So I would say there is room for what is outside of experiment to be held in high esteem based on other considerations. That as I see it, is about the consensus view of scientists, rather than about any law or regularity itself.