Primordial black holes are supposedly astreroid-mass objects leftovers from the Big Bang. According to observation, dark matter makes up 27% of the total mass of the universe as opposed to the 5% of visible matter. Primordial black holes are about the size of a proton and would not be easily detectable. Also, there is something else to consider: Hawking radiation and black holes evaporation.

Black holes that small would only last a few billion years. As they evaporate, they would lose mass and their gravitational pull would lessen and since dark matter is the proverbial glue holding the universe together, everything starts flying apart. Sounds familiar? Computer simulations of galaxies with the effects of dark matter removed fly apart, but why stop there. If primordial black holes fill the void between galaxies, then their evaporation means the gravitational bonds working against the expansion of the universe will diminish giving the appearance of another force accelerating the expansion of the universe a.k.a. dark energy.

https://arxiv.org/abs/1907.05608