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We argue that the striking similarity between the cosmic abundances ofbaryons and dark matter, despite their very different astrophysical behavior,strongly motivates the scenario in which dark matter resides within a rich darksector parallel in structure to that of the standard model. The near cosmiccoincidence is then explained by an approximate $\mathbb{Z}_2$ exchangesymmetry between the two sectors, where dark matter consists of stable darkneutrons, with matter and dark matter asymmetries arising via parallel WIMPbaryogenesis mechanisms. Taking a top-down perspective, we point out that anadequate $\mathbb{Z}_2$ symmetry necessitates solving the electroweak hierarchyproblem in each sector, without our committing to a specific implementation. Ahigher-dimensional realization in the far UV is presented, in which thehierarchical couplings of the two sectors and the requisite$\mathbb{Z}_2$-breaking structure arise naturally from extra-dimensionallocalization and gauge symmetries. We trace the cosmic history, payingattention to potential pitfalls not fully considered in previous literature.Residual $\mathbb{Z}_2$-breaking can very plausibly give rise to the asymmetricreheating of the two sectors, needed to keep the cosmological abundance ofrelativistic dark particles below tight bounds. We show that, despite the needto keep inter-sector couplings highly suppressed after asymmetric reheating,there can naturally be order-one couplings mediated by TeV scale particleswhich can allow experimental probes of the dark sector at high energycolliders. Massive mediators can also induce dark matter direct detectionsignals, but likely at or below the neutrino floor.

A Closer Look in the Mirror: Reflections on the Matter/Dark Matter Coincidence