  We argue that the striking similarity between the cosmic abundances of
baryons and dark matter, despite their very different astrophysical behavior,
strongly motivates the scenario in which dark matter resides within a rich dark
sector parallel in structure to that of the standard model. The near cosmic
coincidence is then explained by an approximate $\mathbb{Z}_2$ exchange
symmetry between the two sectors, where dark matter consists of stable dark
neutrons, with matter and dark matter asymmetries arising via parallel WIMP
baryogenesis mechanisms. Taking a top-down perspective, we point out that an
adequate $\mathbb{Z}_2$ symmetry necessitates solving the electroweak hierarchy
problem in each sector, without our committing to a specific implementation. A
higher-dimensional realization in the far UV is presented, in which the
hierarchical couplings of the two sectors and the requisite
$\mathbb{Z}_2$-breaking structure arise naturally from extra-dimensional
localization and gauge symmetries. We trace the cosmic history, paying
attention to potential pitfalls not fully considered in previous literature.
Residual $\mathbb{Z}_2$-breaking can very plausibly give rise to the asymmetric
reheating of the two sectors, needed to keep the cosmological abundance of
relativistic dark particles below tight bounds. We show that, despite the need
to keep inter-sector couplings highly suppressed after asymmetric reheating,
there can naturally be order-one couplings mediated by TeV scale particles
which can allow experimental probes of the dark sector at high energy
colliders. Massive mediators can also induce dark matter direct detection
signals, but likely at or below the neutrino floor.
