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There is no evidence for small primordial black holes that could make up dark matter.Small black holes (mass of an asteroid) at relativistic speeds passing within 6000 km of Earth could have been detected by LIGO.But since 2015 none have been.

Relativistic black holes Did something stop them forming or did they all evaporate? Black hole detection

Also if small black holes had a high temperature would this show up on the cosmic microwave background? A typical asteroid sized relativistic mass is 50 megatons with a temperature of 122K.

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    $\begingroup$ 6000km is a very very small distance indeed in astronomy, only about 1/6 of the distance to geostationary orbit. Unless we were expecting there to be a truly prodigious number of small black holes, it seems unlikely that any would have passed so close to earth in the last decade. $\endgroup$
    – ScienceSnake
    Commented Sep 4 at 19:25

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Two reasons:

  • Obviously the smaller something is, the harder it is to detect. And small black holes are very small indeed. The mass of the largest asteroids are about $10^{20}$ kg. The corresponding Schwarzschild radius is less than a micron. In other words, you probably can't see it even if it were right in front of you (although you can feel its gravitational attraction).
  • There is almost no dark matter in the Solar System.
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Such a black hole would have a temperature of about 2.5 K and a lifetime considerably shorter than the age of the universe. As such, a population of these objects are not candidates for dark matter, they need to be more massive by at least an order of magnitude and be much colder.

The obvious answer to your headline question is that we may not detect them because they are not there- other candidates for dark matter exist.

If black holes of mass $\geq 10^{15}$ kg were responsible for dark matter, then the local mass density of dark matter of $\leq 0.01 M_{\odot}$/pc$^3$ translates to a number density for those black holes of less than (and potentially, much less than) 1 per 500 cubic Astronomical units.

The paper by Loeb (2024) claims that LIGO would have been capable of detecting a spike-like tidal acceleration, if such objects passed close to the Earth. Even if you accept that LIGO would have detected this- I think that is extremely optimistic at the $10^{14}$ gramme limit proposed, the signal would not be a narrow frequency spike and would last just a few hundredths of a second - the paper suggests an obvious solution to your question: Either such objects are very rare and/or they are not travelling at relativistic speeds.

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  • $\begingroup$ I accept the black holes are rare now but if they were common in the early universe would their high temperature show up in the CMB radiation today. $\endgroup$
    – user57831
    Commented Sep 5 at 8:07
  • $\begingroup$ The claim for detectability by LIGO was in the first reference I provided.The author Avi Loeb says: "Imagine a relativistic object moving near the speed of light within a distance from LIGO that is comparable to the radius of the Earth. At closest approach, such an object would generate a gravitational signal over a time period equal to the radius of the Earth divided by the speed of light. The resulting duration of the signal is 0.02 seconds. Its inverse corresponds to a frequency of 50 hertz, to which LIGO is highly sensitive." $\endgroup$
    – user57831
    Commented Sep 5 at 8:48
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If primordial black holes were small enough to decay away in billions of years (i.e., < 200 million metric tons) and made up a significant share of the dark matter, then the galaxies, groups, and clusters would be disintegrating from the loss of binding mass. Since we observe that galaxies and larger structures like clusters remain stable over time, it indicates that this scenario is not happening.

The epoch of recombination occurs when the temperature of the CMB decreases to the temperature at which protons combine with electrons to form neutral hydrogen. Changes in the energy before recombination would not significantly affect this temperature. However, significant radiation from primordial black holes evaporating over time would increase the photon-to-baryons ratio, which probably would have been noticed. This radiation would also create a background spectrum quite separate in form from the microwave background, which is not the case.

Since these effects are not observed, it suggests that primordial black holes, if they exist as a significant component of dark matter, are not small enough to evaporate within the age of the universe.

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