COSMO In the literature: open

Can asteroid-mass primordial black holes be all the dark matter?

In plain words

Black holes could have formed directly from dense regions in the very early universe. Black holes weighing as much as an asteroid are too small to be seen by most tests, so they could still make up all the dark matter.

Precise statement

For primordial black holes with masses $M = 1e17 \text{ to } 1e22\,\mathrm{g}$, no current constraint excludes a dark-matter fraction $f_{\mathrm{PBH}} = \Omega_{\mathrm{PBH}} / \Omega_c = 1$. Determine whether $f_{\mathrm{PBH}} = 1$ is allowed anywhere in this window, via detection (Hawking evaporation products at the light end, femtolensing or microlensing of stars and gamma-ray bursts, dynamical capture in stars) or exclusion of $f_{\mathrm{PBH}} = 1$ across the whole window for monochromatic and extended mass functions.

What would settle it

Microlensing surveys and MeV $\gamma$-ray observations sensitive enough to probe $f_{\mathrm{PBH}} < 1$ throughout 1e17 to 1e22 g.

Status in the literature

Unverified note

2026 reviews still list $1e17\ \text{to}\ 1e22\,\mathrm{g}$ as a window where $f_{\mathrm{PBH}} = 1$ is allowed.