String theory
(In versions of quantum field theory that include supersymmetry (SUSY), each boson has a corresponding fermion, and vice versa.  That is, each energy particle has a corresponding matter particle, and each matter particle has a corresponding energy particle, yet the unobservable partner is more massive and thus super.  These superpartners might seem an extravagant prediction, yet many theorists and experimentalists favor supersymmetric versions of the Standard Model, whose equations must otherwise be tweaked extravagantly and sometimes arbitrarily to maintain predictive success or mathematical consistency, but with the superpartners align.)