A 9-minute explainer on Yang–Mills theory and QCD: why a theory that predicts the proton's mass still had no mathematical proof that it exists, what "exists" means (infinities, the lattice and its continuum limit, the axioms), confinement and the mass gap, the Clay Millennium problem, and OpenAI's October 26, 2026 claim of an AI-produced existence proof (mass gap not claimed; proofs withheld; not yet independently confirmed).