  Non-Abelian dark gauge forces that do not couple directly to ordinary matter
may be realized in nature. The minimal form of such a dark force is a pure
Yang-Mills theory. If the dark sector is reheated in the early universe, it
will be realized as a set of dark gluons at high temperatures and as a
collection of dark glueballs at lower temperatures, with a cosmological phase
transition from one form to the other. Despite being dark, the gauge fields of
the new force can connect indirectly to the Standard Model through
non-renormalizable operators. These operators will transfer energy between the
dark and visible sectors, and they allow some or all of the dark glueballs to
decay. In this work we investigate the cosmological evolution and decays of
dark glueballs in the presence of connector operators to the Standard Model.
Dark glueball decays can modify cosmological and astrophysical observables, and
we use these considerations to put very strong limits on the existence of pure
non-Abelian dark forces. On the other hand, if one or more of the dark
glueballs are stable, we find that they can potentially make up the dark matter
of the universe.
