12. What the Standard Model does not explain
Measurements and explanations #
An observed effect and a proposed mechanism are different kinds of statement. Neutrino oscillations establish mass-squared splittings; they do not identify a unique mass-generation mechanism. Gravitational dark-matter evidence does not yet identify one confirmed dark-matter particle. An explanation must predict an observable that can distinguish it from alternatives.
12.1 Neutrino mass #
Oscillation data establish nonzero mass-squared splittings, but absolute masses and the Dirac-versus-Majorana character remain incompletely known. This is direct evidence that the minimal Standard Model is incomplete. Review of Particle Physics (2026)
12.2 Dark matter #
Astrophysical and cosmological evidence strongly supports a nonluminous gravitating component, but the Standard Model contains no established particle that reproduces all dark-matter phenomenology. Candidate ideas include axions, weakly interacting massive particles, sterile-neutrino scenarios, and other hidden-sector states. None has yet been confirmed as the dark matter. Review of Particle Physics (2026)
12.3 Matter-antimatter asymmetry #
The observed universe contains far more baryonic matter than antimatter. The Standard Model contains CP violation, but the known amount is insufficient in standard cosmological scenarios to explain the observed baryon asymmetry. Baryogenesis therefore remains an open problem.
It is not correct to say that the early universe is known to have started with exactly equal numbers of matter and antimatter and that the “missing antimatter went into another universe.” That is a speculative construction, not an observational result.
12.4 Gravity and quantum theory #
General relativity and quantum field theory are extraordinarily successful in their tested domains, but a complete experimentally confirmed quantum theory of gravity is absent. Candidate frameworks include string theory, loop quantum gravity, asymptotic safety, causal approaches, effective-field-theory treatments, and others. No single framework has experimental status comparable to the Standard Model.
12.5 Naturalness, hierarchy, and flavor #
The Higgs mass is sensitive to ultraviolet physics in ways that motivate the hierarchy/naturalness problem, although how much weight to assign naturalness criteria is debated. Separately, the observed pattern of Yukawa couplings and mixing angles is simply input into the Standard Model. A future theory may explain that pattern, or it may reveal that our notion of “explanation” needs refinement.