Why do physicists still call the matter-antimatter imbalance unsolved?

What exactly is unsolved?

Observing a matter surplus is different from identifying its origin. CERN explains that measured Standard Model sources of charge–parity asymmetry are insufficient to explain the cosmic imbalance in the usual account. Finding a symmetry violation in a laboratory therefore does not, by itself, finish the explanation. CERN's June 2022 explanation.

A mechanism must supply a quantitative history, not just a reason matter and antimatter could behave differently. When did it operate? How much surplus did it make? Could later reactions remove it? Those questions connect a particle-level idea to an expanding universe.

What are Sakharov's three ingredients?

For conventional generation from an initially balanced state, the usual conditions are: baryon-number-changing reactions; violation of charge conjugation and charge–parity symmetries; and departure from thermal equilibrium. Baryon number tracks baryons minus antibaryons. The first permits changing the balance, the second prevents exact cancellation between corresponding processes, and the third avoids equilibrium undoing the generated surplus. Sakharov's 1967 paper introduced this approach. The conditions are a framework-dependent recipe, not a claim that every imaginable cosmology must use it. Original paper, reprinted 1991.

Ingredients are not sufficient instructions. Having everything listed in a recipe does not establish the quantities, sequence or outcome. Physicists must calculate those details and compare the result with observations. An assumption about the initial state should also be identified rather than silently treated as a derived result.

What are sphalerons?

These concern transitions in electroweak fields, not a newly detected species of particle. The transitions change baryon and lepton numbers together while preserving their difference. They can help convert a lepton imbalance into a baryon imbalance. D'Onofrio and colleagues' 2014 lattice calculations estimate their temperature-dependent rate. The minimal Standard Model's smooth electroweak crossover also makes the usual electroweak-only generation scenario ineffective. A calculated rate supports the theoretical account; it is not a photograph of the early universe. Original sphaleron-rate calculation.

Could neutrinos be responsible?

Leptogenesis is a proposed route. Fukugita and Yanagida's original 1986 model adds right-handed Majorana neutrinos whose out-of-equilibrium decays produce a lepton imbalance. Electroweak processes then convert part of it into a baryon imbalance. These added states and their role in the cosmic history are hypotheses, not established discoveries. Observing ordinary neutrinos does not confirm this mechanism. Original leptogenesis proposal.

Confidence is high that the origin remains open; particular models offer different testable connections. Neutrino measurements can constrain proposed ingredients without uniquely reconstructing that ancient history. Independent evidence for the relevant new states, plus a consistent quantitative abundance calculation and successful additional tests, would change the case. Read the basic leftover-matter puzzle for what the imbalance means; this answer focuses on explaining it.