Why Comparable? A Multiverse Explanation of the Dark Matter-Baryon
  Coincidence
  The densities of dark and baryonic matter are comparable: \zeta = \rho_D /
\rho_B ~ O(1). This is surprising because they are controlled by different
combinations of low-energy physics parameters. Here we consider the probability
distribution over \zeta in the landscape. We argue that the Why Comparable
problem can be solved without detailed anthropic assumptions, and independently
of the nature of dark matter. Overproduction of dark matter suppresses the
probability like 1/(1+\zeta), if the causal patch is used to regulate
infinities. This suppression can counteract a prior distribution favoring large
\zeta, selecting \zeta ~ O(1).
  This effect not only explains the Why Comparable coincidence but also renders
otherwise implausible models of dark matter viable. For the special case of
axion dark matter, Wilczek and independently Freivogel have already noted that
a 1/(1+\zeta) suppression prevents overproduction of a GUT-scale QCD axion. If
the dark matter is the LSP, the effect can explain the moderate fine-tuning of
the weak scale in simple supersymmetric models.
