  We point out that SO($2N$) pure Yang-Mills theory provides a candidate for
dark matter (DM) without the explicit need to impose any additional symmetry.
The DM candidate is a particular type of glueball, which we refer to as a
baryonic glueball, that is naturally stable and produced by a novel production
mechanism for a moderately large $N$. In this case, the intercommutation
probability of cosmic strings (or macroscopic color flux tubes) is quite low,
which offers characteristic gravitational wave signals to test our model. In
particular, our model can simultaneously account for both abundance of DM and
the recently reported gravitational wave signals detected in pulsar timing
array experiments, including NANOGrav.
