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Since 90 % of matter is what we cannot see, why can't it be black-holes from early on? Is is possible to figure out that there are no black holes in the line of sight of various stars/galaxies we observe?

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Even quiescent black holes tend to show up, through microlensing. Observational tests have put pretty rigorous constraints on a range of black holes masses in the Milky Way, although intergalactic black holes are not as well constrained.

The other problem is figuring out how you make lots of black holes, especially at smaller scales. That's not to say that scientists aren't still coming up with ideas, though. Warning: shameless plug. Further warning: We never found any empirical confirmation, so until that happens, it was just a promising idea that didn't pan out.

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    $\begingroup$ Better with references to the papers that provide the "pretty rigorous" constraints and to say what these constraints actually are. $\endgroup$
    – ProfRob
    Aug 25, 2015 at 10:27
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Part of the reason is a matter of ratios.

We're not talking about a fraction of the total mass that can be observed. We're talking multiples. There's about 10x as much dark matter in any large galaxy than there is normal matter that can be observed. If this was all in black holes, then there would be 10x as many black holes as there are stars. That would be pretty obvious actually, as those black holes would interact with other matter in the galaxy (in say, nebulae, both cold and hot), and the effects would be quite obvious and catastrophic. For one, it would make life on earth impossible, as there should be around 10 (give or take a few) black holes hovering around our sun at less than interstellar distances (just by the law of averages), probably spewing x-rays and subatomic particles our way. Nevermind the major gravitational effects they would have on the formation of planets. We're pretty sure that's not happening.

Also, the velocity curves of galaxies where velocities of stars is graphed vs distance from the galactic center, demonstrate that the mass of the galaxy is quite evenly distributed. This would not be the case if the proposed black holes were either exceptionally massive (as with the black hole at the center of most or perhaps all galaxies) or even not-quite-so-massive-but-evenly-distributed.

Instead, dark matter seems to completely lack interaction with other matter except by gravity. This effect can be seen in colliding galaxies, where the galaxies' dark matter halos appear to keep moving while the normal, detectable matter gets left behind.

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    $\begingroup$ it would make life on earth impossible By statistical definition there would be some solar systems out of destructive range of black holes. If observers there extrapolated their perspective as astronomically typical they'd be excluding all the cases of systems that had been eaten, which could be the position we are currently in..? $\endgroup$
    – geotheory
    Feb 7, 2016 at 12:20
  • $\begingroup$ Please. Do keep going with your line of thought and see where it takes you. $\endgroup$
    – Ernie
    Feb 10, 2016 at 22:15
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Paolo Pani and Avi Loeb had two recent papers claiming to rule out the remaining window of primordial black holes as dark matter because they would either distort the CMB or destroy neutron stars: arXiv:1307.5176 and arXiv:1401.3025.

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tl; dr: it can, but needs more observations/theoretical reasons to be convincing.

Black holes can come in a variety of mass ranges, from stellar-mass ones to supermassive black holes. One feature for all black holes however is that they will lens gravity. This effect, called microlensing, is how we can also tell if dark matter can't just be e.g. rogue planets. Obviously, the more massive the black hole, the more likely we are to detect it by microlensing. Microlensing data rules out black holes in a certain mass range. Other constraints are, for example, if the black hole mass is too small, then it would have evaporated by now (via Hawking radiation) and so can't be dark matter.

The result is you end up with something like this image (Fig 2 of paper):

enter image description here

The shaded zones are regions where there is a limit to how much of dark matter can be made of black holes of that mass range, due to various constraints (see caption below for more details).

Compilation of constraints on the PBH fraction (with respect to DM) as a function of the PBH mass, assuming a monochromatic mass function. The different probes considered are: impact of PBH evaporation (red) on the extragalactic γ-ray background (Carr et al., 2010) and on the CMB spectrum (Clark et al., 2017); non-observation of microlensing events (blue) from the MACHO (Alcock et al., 2001), EROS (Tisserand et al., 2007), Kepler (Griest et al., 2014), Icarus (Oguri et al., 2018), OGLE (Niikura et al., 2019b) and Subaru-HSC (Croon et al., 2020) collaborations; PBH accretion signatures on the CMB (orange), assuming spherical accretion of PBHs within halos (Serpico et al., 2020); dynamical constraints, such as disruption of stellar systems by the presence of PBHs (green), on wide binaries (Monroy-Rodríguez and Allen, 2014) and on ultra-faint dwarf galaxies (Brandt, 2016); power spectrum from the Lyα forest (cyan) (Murgia et al., 2019); merger rates from gravitational waves (purple), either from individual mergers (Kavanagh et al., 2018; Abbott et al., 2019) or from searches of stochastic gravitational wave background (Chen and Huang, 2020). Gravitational waves limits are denoted by dashed lines, since they could be invalidated (Boehm et al., 2021). Dotted brown line corresponds to forecasts from the 21 cm power spectrum with SKA sensitivities (Mena et al., 2019) and from 21 cm forest prospects (Villanueva-Domingo and Ichiki, 2021). Figure created with the publicly available Python code PBHbounds (Kavanagh, 2019).

[PBH stands for Primordial Black Hole. I'll discuss them in more detail later, but for now they are effectively just black holes.]

So although some number of solar-mass black holes exist, they aren't nearly as numerous to account for all dark matter.

However! Note there is a window - around the $10^{17}-10^{22} g$ mass ranges - where black holes aren't constrained. They can therefore account for all dark matter. However! This mass range is well below the minimum mass of a star (the Earth's mass is also above this range), which begs the question of how the black hole formed.* The only reasonable alternative is, I believe, that they are primordial - that is, they formed from density perturbations in the very early universe. If you've not encountered primordial black holes before, they're something of a unicorn in cosmology - a last-resort explanation - because they work whenever you need a missing mass. In turn, this is because the primordial power spectrum during inflation (i.e., the magnitude of the density fluctuations that ultimately gave rise to large-scale structures like galaxies) is not well-constrained.

So we come full circle. It is possible that dark matter is 100% black holes, but that statement is not convincing unless 1) we find lots of such black holes [not likely given how small they are] or 2) we derive from theoretical considerations how inflation is expected to produce black holes in this mass range, and in sufficient quantities to explain dark matter. Until then - we just don't know.

*There's a separate constraint on the total amount of baryonic dark matter due to the CMB. If any of these black holes are astrophysical in origin, then they must have formed from baryons, and therefore cannot be all dark matter.

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    $\begingroup$ Happy to see an updated response to this. One comment: if the dark matter were black holes, they would have to be primordial no matter the mass. Any black hole of astrophysical origin was previously gas, and before that, part of the primordial plasma of the early universe. But we know from the cosmic microwave background and the distribution of elements that emerged from primordial nucleosynthesis that the dark matter today was also dark matter in the early universe. $\endgroup$
    – Sten
    Feb 6, 2023 at 10:22
  • $\begingroup$ Also, the usual primordial black hole formation mechanism creates them as a result of inflationary dynamics but not during inflation. The idea is that perhaps inflation seeded some initial density variations of particularly high amplitude. When a region of extreme excess density (~twice as dense as the cosmological average) becomes smaller than the cosmological horizon, at potentially a much later time, it promptly collapses into a black hole. $\endgroup$
    – Sten
    Feb 6, 2023 at 10:29
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    $\begingroup$ @Sten thanks, I've updated the answer. If you're an expert (you sure sound like one), you should write your own answer as well =) $\endgroup$
    – Allure
    Feb 7, 2023 at 8:04
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Black holes are relatively easy to see as they emit high energy radiation.

And even when there is nothing falling into a black hole to produce radiation, light is bent round a black hole, so makes it visible.

Dark matter isn't easy to see, in fact it is incredibly hard to identify - which is why the debate continues as to how much there is.

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    $\begingroup$ I'm pretty sure that's an artist's concept rendering with more than one flaw. To name a few: the beams normal to the disk are supposedly Hawking radiation, which wouldn't be visible from a side view unless they were interacting with some other matter (gas cloud, etc.); the accretion disk is only visible while the black hole is "feeding" (i.e. matter is actively falling into it); we wouldn't see a hole in the middle even while it's feeding since the light from the accretion disk would bend around the surface -- we'd see a star-like object. $\endgroup$
    – villageidiot
    Apr 21, 2012 at 3:30
  • $\begingroup$ I agree that it's not the best visual aid, but I think @RoryAlsop's argument is sill valid. $\endgroup$
    – Hassan
    Apr 24, 2012 at 0:15
  • $\begingroup$ I removed the deliberately over the top artists impression :-) $\endgroup$
    – Rory Alsop
    Apr 24, 2012 at 7:42
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    $\begingroup$ @villageidiot The beams are most assuredly not Hawking radiation. Those are the "jets" and they form in the accretion disk, not in or near the black hole themselves so no exotic physics (beyond magnetohydrodynamics) is needed. $\endgroup$ Apr 24, 2012 at 17:48
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Smaller black holes (size of the moon) are not so easily detectable.

And if you think it's so improbable that we could have black holes "near" earth and remain undetected, then the same could be said for a dark matter "particle". Seems much more likely to me that dark matter is something we know and have seen (matter, atoms) than something "new" since it apparently makes up 5/6ths of the mass of the universe.

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    $\begingroup$ The micro-lensing MACHO searches conducted during and since the 1990s tightly constrain the density and masses of unobserved black holes. They are not ruled out, but we actually have pretty good evidence that they are few and far between. $\endgroup$ Feb 18, 2014 at 1:22

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