The Second Flavor of hydrogenic atoms/ions and their applications in atomic physics, nuclear physics, and cosmology

We overview the second solution of the Dirac equation for hydrogen atoms that became legitimate with the allowance for the actual charge density distribution inside the protons. It led to the possible existence of hydrogen atoms having only the S-states and therefore not interacting with the electromagnetic radiation – remaining dark . The actual existence of such Second Flavor of Hydrogen Atoms (SFHA) is evidenced by three different types of atomic experiments. In addition to their application for explaining the atomic experiments, they were the basis for resolving the long-standing puzzle of the neutron lifetime. Also, the SFHA was the basis for explaining the baffling astrophysical observation of the anomalous absorption in the 21 cm line from the early Universe, what made the SFHA the leading candidate for baryonic dark matter. Further, by comparing astrophysical observations with atomic experiments, it was demonstrated that the SFHA constitutes the majority of baryonic dark matter. Lastly, it was shown that neutron stars slowly but continuously generate new baryonic dark matter in the form of the SFHA and that there is astrophysical evidence of this process.