Review of dark hydrogenic atoms and ions as baryonic dark matter

From astrophysical observations, the Planck Collaboration calculated the ratio of baryonic dark matter (DM) to the non-baryonic DM as 1:5. We showed: most of the baryonic DM is the second flavor of hydrogen atoms (SFHA). The SFHA are dark atoms: they have only the S-states. According to the quantum selection rules, they do not interact with electromagnetic radiation. Theoretically, the SFHA is based on the second solution of the standard quantum Dirac equation outside the proton of its actual size and actual charge density distribution (CDD). The existence of the SFHA is evidenced by several different types of atomic experiments, by an astrophysical observation, and by its ability to resolve the long-standing puzzle of neutron lifetime. Some heavy hydrogenic ions, whose nuclei are, e.g. [Formula: see text]Ca[Formula: see text], [Formula: see text]Ca[Formula: see text], can also have the dark (second) flavor. In calcium–rich supernovae (CRS), the conditions are favorable for the production and survival of dark hydrogenic calcium ions, whose possibility to exist in laboratory experiments and elsewhere was predicted previously. Thus, CRS can generate new baryonic DM in the form of dark hydrogenic calcium ions. This would be the secondary mechanism for generating new baryonic DM, the primary mechanism being the production of the SFHA by neutron stars.