I read that the expansion of our universe is the expansion of space itself. Apparently, the phrase, expansion of space itself, is meant to characterize the observed displacements of very large scale cosmological structures as an artifact of the isotropic stretching or expanding or some unknown physical structure called "space" (thus continually creating new space), and to distinguish this sort of intrinsic expansion from an isotropic stretching or expansion of large scale cosmological structures into an already existing space. I imagine that the very large scale isotropic expansion (and therefore also the very large scale structure of the universe) is due to kinetic energy imparted during some sort of (extrinsic) Big Bang event and not a function of (intrinsic) wave interaction, and that at smaller scales it is wave interaction intrinsic to our universe (in the forms of gravity, electricity, magnetism, weak force, strong force, etc.) that is the structural determinant. My question is, if the ontological status of "space" is unknown, then how is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?