Number and Entropy of Halo Black Holes
  Based on constraints from microlensing and disk stability, both with and
without limitations from wide binary surveys, we estimate the total number and
entropy of intermediate mass black holes. Given the visible universe comprises
$10^{11}$ halos each of mass $\sim 10^{12} M_{\odot}$, typical core black holes
of mean mass $\sim 10^7 M_{\odot}$ set the dimensionless entropy ($S/k$) of the
universe at a thousand googols. Identification of all dark matter as black
holes sets the dimensionless entropy of the universe at ten million googols,
implying that dark matter can contribute over 99% of entropy, which favors all
dark matter as black holes in the mass regime of $ \sim 10^{5} M_{\odot}$.
