Dark Atoms and Dark Radiation

We have invited investigators to contribute original research and review
articles that seek to define the possible physical nature of composite dark
matter and its constituents, to stimulate the experimental searches and
to suggest astrophysical tests for their effects. The collected papers in this
special issue represent only a first small step in the approach to the thorough
investigation of the topic of dark atoms and dark radiation, demonstrating
the wide but far from complete variety of possible aspects of this problem.