Dark Hydrogen Atoms as Baryonic Dark Matter

We briefly review the Second Flavor of Hydrogen Atoms (SFHA) that are dark hydrogen atoms emerging from atomic theory and atomic experiments. Specifically, their existence is supported by three different kinds of atomic experiments; besides, it resolved the long-standing puzzle of the lifetime of free neutrons. The SFHA-based explication (both qualitative and quantitative) of the observed anomalous absorption signal in the redshifted 21 cm line from the early Universe highlighted the SFHA as a compelling candidate for baryonic Dark Matter (DM) - because the alternative explanations were either invalidated in ensuing publications by other authors, or resorted to exotic, never-discovered subatomic particles and/or to changing the laws of physics. In contrast, the SFHA is based on the standard quantum mechanics (the Dirac equation): it does not concoct anything beyond the Standard Model and does not resort to changing the laws of physics. For this reason, the SFHA-based explication of the observed anomalous absorption signal in the redshifted 21 cm line is favored by the Occam razor principle. We also clarify the confusing employment of the terminology “dark hydrogen atoms” with respect to exotic hypothetical objects beyond the Standard Model. We demonstrate that some of this objects are not actually dark, while others were claimed to be dark simply because the word “dark” was arbitrarily attached to their names. We also touched upon the socalled “small hydrogen atoms” and demonstrate that this is a fallacy. Also, by comparing the pertinent astrophysical ratio (determined from observations) to the corresponding ratio from the atomic experiments, we showed that the SFHA seems to currently represent most of baryonic DM. Finally, as we showed that in the two-body decay channel of the neutron decay the outcome should be overwhelmingly the SFHA rather than the usual hydrogen atoms, this leads to the conclusion that neutron stars - in three different situations - gradually generate new baryonic DM in the form of the SFHA, and that there is an (indirect) astrophysical evidence of such generation.