Mirror Nucleosynthesis and Diffuse Dark Matter (XVI v4)

Building on the nucleon mode tables and leptonic structure established in Parts XIII–XV of the FCD-RB series, where the visible proton mirrors an antineutron on the mirror brane and the visible neutron mirrors an antiproton, we show that the mirror brane carries an inverted baryon ratio ¯p/¯n ≈ 1 : 7 at the end of the Big Bang epoch. This paper formalizes primordial nucleosynthesis under this severe antiproton-sparse constraint. We propose a statistical Pauli blocking mechanism in the early thermal bath that stabilizes free antineutrons against free decay, allowing mirror nucleosynthesis to proceed with a strongly antineutron-dominated composition. The resulting mirror nuclear chemistry proceeds through antideuteron and antitritium intermediate steps to consume all available antiprotons, leaving approximately 50% anti-helium-4 and 50% free antineutrons (or antitetraneutrons) by mass, with absolutely zero mirror hydrogen. This unique composition leads to a catastrophic cooling crisis below 10, 000 K due to the absence of the molecular H 2 cooling channel, freezing the fragmentation Jeans mass at galactic scales (10 6 − 10 7 M ⊙ ) and preventing the formation of mirror stars or compact objects. This provides a natural, parameter-free resolution to the cusp-core, missing satellites, and too-big-to-fail problems of Cold Dark Matter (CDM). We predict a two-component dark-matter halo structure (inner anti-He-4 and outer diffuse antineutrons) and a cross-brane primordial nucleosynthesis relation f(4 He, mirror) ≈ 2Y p ≈ 0.49.