  We point out a unique mechanism to produce the relic abundance for glueball
dark matter from a gauged $SU(N)_d$ hidden sector which is bridged to the
standard model sector through heavy vectorlike quarks colored under gauge
interactions from both sides. A necessary ingredient of our assumption is that
the vectorlike quarks, produced either thermally or non-thermally, are abundant
enough to dominate the universe for some time in the early universe. They later
undergo dark color confinement and form unstable vectorlike-quarkonium states
which annihilate decay and reheat the visible and dark sectors. The ratio of
entropy dumped into two sectors and the final energy budget in the dark
glueballs is only determined by low energy parameters, including the intrinsic
scale of the dark $SU(N)_d$, $\Lambda_d$, and number of dark colors, $N_d$, but
depend weakly on parameters in the ultraviolet such as the vectorlike quark
mass or the initial condition. We call this a cosmic selection rule for the
glueball dark matter relic density.
