  Our understanding of the Universe is known to be incomplete and new gauge
forces beyond those of the Standard Model might be crucial to describing its
observed properties. A minimal and well-motivated possibility is a pure
Yang-Mills non-Abelian dark gauge force with no direct connection to the
Standard Model. We determine here the relic abundances of the glueball bound
states that arise in such theories and investigate their cosmological effects.
Glueballs are first formed in a confining phase transition, and their relic
densities are set by a network of annihilation and transfer reactions. The
lightest glueball has no lighter states to annihilate into, and its yield is
set mainly by 3 to 2 number-changing processes which persistently release
energy into the glueball gas during freeze-out. The abundances of the heavier
glueballs are dominated by 2 to 2 transfer reactions, and tend to be much
smaller than the lightest state. We also investigate potential connectors
between the dark force and the Standard Model that allow some or all of the
dark glueballs to decay. If the connection is weak, the lightest glueball can
be very long-lived or stable and is a viable dark matter candidate. For
stronger connections, the lightest glueball will decay quickly but other
heavier glueball states can remain stable and contribute to the dark matter
density.
