Primordial Black Hole
Black holes can, in theory, be of any size, but they all require extraordinary energy and density conditions to form. Those conditions were present in the early moments of the universe, leading to speculation among physicists that some small black holes could have formed — which is why they are called primordial.
Because every black hole sits on the Schwarzschild line, a primordial black hole is fixed by one number, its mass. If they exist, they would be invisible except through their gravity, which makes them a candidate for dark matter: no new particle needed, only black holes made in the first second. The highlighted stretch of the line is the area of possible PBH, the masses at which they could still be all of the dark matter — roughly 10^{17} to 10^{22} g, the mass of a mountain to that of a large asteroid, squeezed to somewhere between a thousandth of an atom and a small virus. Two kinds of search close it in from either side:
Hawking radiation (the lower limit). A black hole slowly radiates, and the smaller it is, the hotter and faster: one of about 5 \times 10^{14} g, formed at the Big Bang, would just be finishing its evaporation today, so lighter ones are already gone. Heavier ones up to about 10^{17} g would still be shining, in gamma rays, X-rays and electron–positron pairs. Satellites measuring the gamma-ray background, the 511 keV glow from positrons in our galaxy, the Voyager probes counting electrons beyond the heliosphere, and the cosmic microwave background all see far less than a dark-matter-sized population of them would give off. So below about 10^{17} g they can be at most a small fraction of the dark matter.
Microlensing (the upper limit). A black hole passing in front of a star bends its light and makes it briefly brighten. Subaru's Hyper Suprime-Cam watched tens of millions of stars in the Andromeda galaxy for a night and found almost none of the short brightenings that black holes from about 10^{22} g up would cause; the MACHO, EROS and OGLE surveys of the Magellanic Clouds and the galactic bulge, and Kepler, close the heavier masses up to tens of Suns. Below about 10^{22} g a black hole is smaller than the wavelength of the light and the star's own disk, and the lensing washes out.
Between the two, the asteroid-mass window is still open: no observation yet rules out that all the dark matter is primordial black holes of about 10^{17}–10^{22} g. Proposed tests include X-ray microlensing, the capture of such black holes by stars and neutron stars, and their gravitational tug on spacecraft.
Sources: Carr, Kohri, Sendouda & Yokoyama, "Constraints on primordial black holes", Rep. Prog. Phys. 84, 116902 (2021); Niikura et al., "Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations", Nature Astronomy 3, 524 (2019); Montero-Camacho et al., "Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates", JCAP 08, 031 (2019).
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