Large $N$-ightmare Dark Matter
  A dark QCD sector is a relatively minimal extension of the Standard Model
(SM) that admits Dark Matter (DM) candidates but requires no portal to the
visible sector beyond gravitational interactions: A "nightmare scenario" for DM
detection. We consider a secluded dark sector containing a single flavor of
light, vector-like dark quark gauged under $SU(N)$. In the large-$N$ limit,
this single-flavor theory becomes highly predictive, generating two DM
candidates whose masses and dynamics are described by few parameters: A light
quark-antiquark bound state, the dark analog of the $\eta'$ meson, and a heavy
bound state of $N$ quarks, the dark analog of the $\Delta^{++}$ baryon. We show
that the latter may freeze-in with an abundance independent of the confinement
scale, forming DM-like relics for $N \lesssim 10$, while the former may
generate DM via cannibalization and freeze-out. We study the interplay of this
two-component DM system and determine the characteristic ranges of the
confinement scale, dark-visible sector temperature ratio, and $N$ that admit
non-excluded DM, once the effects of self-interaction constraints and bounds on
effective degrees of freedom at the BBN and CMB epochs are included.
