  Primordial black holes (PBHs) could provide the dark matter in various mass
windows below $10^2 M_{\odot}$ and those of $30 M_{\odot}$ might explain the
LIGO events. PBHs much larger than this might have important consequences even
if they provide only a small fraction of the dark matter. In particular, they
could generate cosmological structure either individually through through the
`seed' effect or collectively through the `Poisson' effect, thereby alleviating
some problems associated with the standard CDM scenario. If the PBHs all have a
similar mass and make a small contribution to the dark matter, then the seed
effect dominates on small scales, in which case PBHs could seed the
supermassive black holes in galactic nuclei or even galaxies themselves. If
they have a similar mass and provide the dark matter, the Poisson effect
dominates on all scales and the first bound clouds would form earlier than in
the usual scenario, with interesting observational consequences. If the PBHs
have an extended mass spectrum, which is more likely, they could fulfill all
three roles - providing the dark matter, binding the first bound clouds and
generating galaxies. In this case, the galactic mass function naturally has the
observed form, with the galaxy mass being simply related to the black hole
mass. The stochastic gravitational wave background from the PBHs in this
scenario would extend continuously from the LIGO frequency to the LISA
frequency, offering a potential goal for future surveys.
