This line of thinking led me to a paper I had not read before (even though it has a tremendous number of citations and is reference (2) in his famous Black Hole Explosions? (evaporation) letter of 1974[1]), by Hawking (1971) <https://academic.oup.com/mnras/article/152/1/75/2604549> (PDF freely available there).

In this paper he considers the possibility that the universe is almost dominated by tiny (~ micrograms) black holes formed in the very early universe, persisting to the 1970s and leaving tracks in sufficiently large and sufficiently numerous cloud chamber detectors given sufficient time. (This was also decades before optical detectors like at SNOLAB, but incongruously after the Nobel prize for bubble chambers, although it was on the cusp of the activation of <https://en.wikipedia.org/wiki/Gargamelle> and the Big European Bubble Chamber -- he would have been aware of how close they were to activation, and results from smaller predecessors). Fascinatingly, several years after publication, conceptually similar papers were put forward for hunts for dark matter, even though it is impossible to extract from this Hawking 1971 paper that he was thinking of galactic rotation curves.

The paper also pre-dates evidence for the acceleration of the expansion of the universe, and contains the curious sentence: "An upper bound on the number of these objects can be set from the measurments by Sandage (7) of the deceleration of the expansion of the Universe." However, the excellent Allan Sandage paper of 1961 <https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_que...> does not really let one take sides ("It suffices to say that no uncontested evidence is presently available [to choose between steady-state and expanding universe]") like Hawking 1971 seems to. Nevertheless, Hawking raises an interesting and slightly spooky premonition of dark matter at super-galactic scale, keeping clusters of galaxies looking like clusters over long periods of time, rather than separating into individual isolated galaxies: "This extra density could stabilize clusters of galaxies which, otherwise, appear mostly not to be gravitationally bound." In context Hawking was clearly thinking about a universe that grows into some very long term non-expanding/non-contracting configuration which includes clumps of galaxies.

Hawking 1971 tl;dr is "good luck actually observing these small weakly-interacting things, if they exist". Which is appropriate for 21st century particle dark matter experimentalists too. :^)

Penultimately, some patience with google scholar has found that a small handful of the many articles citing Hawking 1971 explore the idea of primordial black holes as dark matter. (but see below).

And finally, sometimes asking questions like yours (how does one build a very small black hole?) and taking them seriously leads to some very early, vague, ideas about how the universe might work in bulk, which is probably very very similar to what provoked Hawking's letter about 52 years ago.

- --

[1] https://sci-hub.se/https://doi.org/10.1038/248030a0

The relevant part of his evaporation letter: "There might, however, be much smaller black holes which were formed by fluctuations in the early Universe². Any such black hole of mass less than 10^15 g would have evaporated by now." In that context, I think a very low motivation for someone not terribly interested in the history of primordial black hole research to have previously hunted down a 1971 note whose idea was shortly thrown under the bus by its author is fairly understandable. :^)

Hawking 1971 is however an early note on primordial black holes (the only obvious earlier one in the literature is from Zel'dovich and Novikov in 1966), and that seems to be behind many of the citations I briefly sampled. Practically all also cite the more extensive follow-up Carr & Hawking 1974 "Black holes in the early Universe" <https://ui.adsabs.harvard.edu/abs/1974MNRAS.168..399C/abstra...> which is very close in submission date to the publication date of Hawking's "Black hole explosions?" yet does not appear to consider evaporation.