  The Hawking process results in a monotonic decrease of the black hole mass,
but a biased random walk of the black hole angular momentum. We demonstrate
that this stochastic process leads to a significant fraction of primordial
black holes becoming extremal Kerr black holes (EKBHs) of one to a few Planck
masses regardless of their initial mass. For these EKBHs, the probability of
ever absorbing a photon or other particle from the cosmic environment is small,
even in the cores of galaxies. Assuming that EKBHs are stable, they behave as
cold dark matter, and can comprise all of the dark matter if they are formed
with the correct initial abundance.
