We consider the possibility that massive primordial black holes are the dominant form of dark matter. Black hole formation adds a Poisson noise to the matter density fluctuations. We use Lyα forest observations to constrain this Poisson term, which constrains the black hole masses to be less than approximately a few times 10~4 solar mass. We also find that structures with less than ～10~3 black holes evaporate by now.
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