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Suppose that there are many primordial black holes. These are point masses which have a small probability to collide with stars let alone themselves.

They are formed in the early universe and could have served as the dark matter guiding the formation of galaxies. They are the ultimate form of dark matter, only interacting by gravity.

If they are indeed present then how would the stars look? Would they disturb our vision by a lensing effect?

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The answer is no, because these effects have been looked for and not found. The microlensing surveys of the 80s and 90s specifically set out to look for the lensing signatures of compact, massive objects, including black holes.

Whilst some lensing events were seen, the numbers were low enough that it could be concluded that there was no significant dark matter in the form of compact halo objects with masses in the range $6\times 10^{-8} < M/M_\odot < 15$ (Tisserand et al. 2008).

That does not mean that there cannot be lots of primordial black holes. It just means that they are either of too low mass or too rare to produce many microlensing events. And don't forget, these were microlensing studies pointed towards very dense stellar fields to maximise the chances of those microlensing events being seen.

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    $\begingroup$ Dòes this mean that dark matter could consist as a halo primordial holes? $\endgroup$
    – StarsratS
    Jul 4 at 13:57
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    $\begingroup$ @StarsratS I'm not sure I'd go that far, but the microlensing surveys could not rule that out so long as they were outside the mass range I've indicated. $\endgroup$
    – ProfRob
    Jul 4 at 14:05
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    $\begingroup$ @StarsratS it implies that IF dark matter exists as halo primordial black holes, then they must have masses outside of the mass range they were not found in. $\endgroup$ Jul 4 at 14:35

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