Image 1 Caption:

observable universe

primordial black holes (PBHs)

Features:

Note, Image 1 is very MISLEADING. The PBHs that may make up some or all of dark matter are NOT the central supermassive black holes (SMBHs, mass range ∼ 10**5--10**10 M_☉) of galaxies though the SMBHs may have had progenitors that were PBHs. The central SMBHs are far too small total mass though individually they are the most massive black holes. The PBHs if they exist and makeup a significant fraction of dark matter must be much less massive individually and form a centrally concentrated distribution thoughout dark matter halos and a more spread-out distribution throughout the intergalactic medium (IGM).
As aforementioned, PBHs are a possible form of dark matter. However, if they exist, they must have formed before ∼ 1 s after the fiducial cosmic time zero (i.e., t=0, lookback time 13.797(23) Gyr (Planck 2018))) of the Friedmann equation models (Wikipedia: Primordial black hole: Formation). This cosmic time was before the Big Bang nucleosynthesis era (cosmic time ∼ 10--1200 s ≅ 0.17--20 m) which they CANNOT have affected significantly if they exist.
We will NOT fully explicate Image 1 here, but we do list some keywords:
We will elaborate on the keyword astroidal-mass primordial black holes (PBH, ∼ 10**17--10**23 g) which have NOTHING to do with asteroids, except they have masses comparable to PBHs that may make up the dark matter (see the next item).
Various observations (including those of gravitational microlensing of stars by various possible astronomical objects (i.e., black holes, brown dwarfs, dim stars, neutron stars, white dwarfs)), suggest that if PBHs make up all or nearly all dark matter, their masses must lie in the asteroid window for primordial black hole masses (∼ 10**17--10**23 g) (see, e.g., Auffinger 2022; Wikipedia: Primordial black holes: Observational limits and detection strategies).
Such astroidal-mass PBHs (∼ 10**17--10**23 g) have very small Schwarzschild radii: note,
```
R_sch = 2GM/c**2 in general

      = 1.4851 Å [M/(10**20 g)] for middle of the asteroidal mass range for PBHs
      = 0.14851 μm [M/(10**23 g)] for high end of the asteroidal mass range for PBHs

      = 2.9532 km (M/M_☉)
      ≅ 3 km (M/M_☉)
 
      = 0.019741 AU [M/(M_☉**6)]
      ≅ 0.02 AU [M/(M_☉*10**6)]

      = 19.741 AU [M/(M_☉**9)]
      ≅ 20 AU [M/(M_☉*10**9)]

      = 9.5741*10**(-5) pc [M/(M_☉*10**9)]
      ≅ 10**(-4) pc [M/(M_☉*10**9)]  .  
```
Note, astroidal-mass PBHs (∼ 10**17--10**23 g) have microscopic event horizons.
Given that they are so small individually, there must be a lot of them if they make up all dark matter. If they all have mass 10**19 g (implying R_sch = 0.14851 Å which is sub-atomic), then their mean separation in the Milky Way is ∼ 15 astronomical units (AU). See What is the space density of astroidal black holes in the Solar System?.
There may be one astroidal-mass PBH (∼ 10**17--10**23 g) in the Solar System out to the Jupiter orbit (mean orbital radius = 5.2038 AU) at any time, but this estimate is very uncertain. See What is the space density of astroidal black holes in the Solar System?.
It seems odd that we CANNOT tell whether dark matter is exotic hypothetical particles (i.e., microscopic scale particles that are NOT baryonic matter and interact with each and baryonic matter very weakly, except via gravity) or macroscopic scale primordial black holes (PBHs) in asteroid window for primordial black hole masses (∼ 10**17--10**23 g). But we CANNOT. All we see is the gravity effect of some sort of zero (or nearly zero) pressure matter entities (that are NOT baryonic matter) that clumps to form dark matter halos. There is NO limits on the masses of the exotic hypothetical particles and only the asteroid window for primordial black hole masses (∼ 10**17--10**23 g) may limit the masses of PBHs.
Image 2 Caption: A collage image showing the comparative sizes of 9 asteroids. For more explication of Image 2, see Asteroid file: asteroid_collage.html.
In order to make the asteroid window for primordial black hole masses (∼ 10**17--10**23 g) somewhat intelligible, we consider the masses of asteroids below with the masses of Ceres (NOT shown in Image 2), Vesta, and Itokawa as examples.
To make the asteroid window for primordial black hole masses (∼ 10**17--10**23 g) somewhat intelligible, we can write a fiducial value formula for asteroid mass:
M_asteroid = [(4π/3)*R**3]*ρ = [(1.57 ...)*10**21 g]*[(D/100 km)**3]*[ρ/(3 g/cm**3)] ,
where R is mean radius, D is mean diameter, ρ is mean density, 100 km is a fiducial value for an asteroid mean diameter, and 3 g/cm**3 is a fiducial value for a rocky asteroid mean density.
For rocky asteroids, the mean density does NOT vary much from 3 g/cm**3. For rocky-icy asteroids with ∼ 50% rock and ∼ 50% water ice by mass, the mean density does NOT vary much from 2 g/cm**3.
The mean diameters and hence the masses of asteroids do vary widely. Table: Asteroid Mass below gives some example asteroid masses from established measurements (the M values) and those calculated from the fiducial value formula (the M_fid values).
We see that the fiducial value formula is better than factor of 2 accurate when compared to the masses of real asteroids. The main reason for the disagreement between the M values and M_fid values is density. Recall, all M_fid values were calculated with the fiducial value ρ = 3 g/cm**3. To explicate:
1. Ceres has actual mean density 2.162(8) g/cm**3. This low value is probably because Ceres is 1/4 water ice by mass (see Wikipedia: Ceres: Geology).
2. Vesta has actual mean density 3.456(35) g/cm**3. This value a bit higher than our fiducial value just because Vesta has somewhat denser rock on average than indicated by our fiducial value.
3. Itokawa has actual mean density 1.95(14) g/cm**3. This low value is primarily because Itokawa is a rubble pile asteroid which means there are cavities in its interior (see Wikipedia: 25143 Itokawa: Physical characteristics).
We also see that our fiducial values for mean diameter and mean density give a fiducial value mass 1.57*10**21 g that is larger than the logarithmic mean mass of the asteroid window for primordial black hole masses (∼ 10**17--10**23 g) which is (17+23)/2 = 20 (i.e., in antilogarithm form mass 10**20 g) by more than 1 dex (i.e., in antilogarithm form by more than a factor of 10).

Images:

Credit/Permission: © European Space Agency (ESA), 2021 (uploaded to Wikimedia Commons by User:Dabed, 2021) / Creative Commons CC BY-SA 3.0.
Image link: Wikimedia Commons: File:PBHs-formation.png.
Credit/Permission: NASA / JPL-Caltech / JAXA / ESA, 2011 (uploaded to Wikipedia by User:Eumeldingens, 2011) / Public domain.
Image link: Wikipedia: File:Asteroidsscale.jpg.

local link: black_hole_primordial.html

Black hole file

black_hole_primordial.html

M_asteroid = [(4π/3)*R**3]*ρ = [(1.57 ...)*10**21 g]*[(D/100 km)**3]*[ρ/(3 g/cm**3)] ,

M_asteroid = [(4π/3)*R**3]ρ = [(1.57 ...)10**21 g]*[(D/100 km)**3]*[ρ/(3 g/cm**3)] ,