  We discuss the relationship between dark matter and the entropy of the
universe with the premise that dark matter exists in the form of primordial
black holes (PBHs) in a hierarchy of mass tiers. The lightest tier are
intermediate-mass PIMBHs within galaxies including the Milky Way. Supermassive
black holes at galactic centres are in the second tier. We are led to speculate
that there exists a third tier of extremely massive PBHs, more massive than
entire galaxies. We discuss future observations by the Rubin Observatory and
James Webb Space Telescope.
