  The first gamma-ray line originating from outside the solar system that was
ever detected is the 511 keV emission from the center of our Galaxy. The
accepted explanation of this signal is the annihilation of electrons and
positrons. However, despite 30 years of intense theoretical and observational
investigation, the main sources of positrons have not been identified up to
now. In this paper we propose an alternative explanation: the observed signal
is due to atomic transitions to "small hydrogen atom," where electron is
captured by proton on a small tight orbit around proton. We describe the status
of the experimental search to find the small hydrogen atom both in astrophysics
data and the lab, and propose new methods how to discover it in the lab
directly. We also propose a search for evidence of the hyperfine splitting due
to small hydrogen. We also discuss a question if the small hydrogen could be
one type of Dark Matter.
