Metaphysics

Metaphysics is anything related to the "physics" of physics, i.e. physics is a certain way because of some higher metaphysical law. In popular culture, it's often misconstrued to mean anything spiritual or related to psionics because those topics assume certain metaphysical laws not assumed by modern science, such as physics being the product of a higher spiritual plane. Metaphysics in this sense also refers to any scientific theories which attempt to explain why phenomena act a certain way, such as why gravity causes attraction.

Provability
Metaphysical (or causal) theories are fundamentally incapable of being undoubtedly proven because doing so requires absolute knowledge of everything. For instance, one might theorize that gravity is caused by mass' effect on a non-euclidean plane, but at any given point more evidence might invalidate this claim. The unknown answers to questions that aren't themselves even known open up the possibility for just about any explanation of causality, from the serious to the outright bizarre or even illogical. For this reason, scientific metaphysical theories are used as models, imperfect mathematical approximations, rather than absolute explanations or truths.