Astronomers predict that within the first seconds after the Big Bang, some black holes were able to form due to an inhomogeneous spatial distribution of matter, and these black holes are called primordial black holes (PBHs).
Some theoretical physicists believe that the inability of primordial black holes to be directly observed makes them well suited as candidates for dark matter.
Most of the dark matter may be primordial black holes.
However, the gravitational wave observation statistics limit this possibility.
Others believe that clusters formed by a large number of primordial black holes may be able to bypass the limits of gravitational wave observations.
A recent paper published in Physical Review Letters has again ruled out the possibility that dark matter is a cluster of primordial black holes.
Researchers say that for stellar-mass primordial black holes, even if they could avoid the observational limits of microgravitational lensing when clustered together in large numbers, this would violate the limits of the Lyman α forest again.
The Lyman α forest is a dense region of absorption lines in the spectrum of high-redshift quasars located on the short-wave side of the Lyman α emission line.
It is caused by the absorption of intergalactic hydrogen clouds.
The primordial black hole cluster would cause an observable change in the observed data of the Lyman α forest, but we do not observe such a change.
So dark matter cannot be primordial black holes.