One of the seven Millennium Prize Problems, announced in 2000 with a 1,000,000-dollar prize, and still unsolved.
The official statement: prove that for any compact simple gauge group G, a non-trivial quantum Yang–Mills theory exists on R⁴ and has a mass gap Δ > 0.
Experiment and computer simulations suggest the mass gap exists in the quantum Yang–Mills theories underlying the strong interaction, but no rigorous proof is known.