# Why can't dark matter be black holes?

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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Better with references to the papers that provide the "pretty rigorous" constraints and to say what these constraints actually are. – Rob Jeffries Aug 25 '15 at 10:27

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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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..? – geotheory Feb 7 at 12:20
Please. Do keep going with your line of thought and see where it takes you. – Ernie Feb 10 at 22:15

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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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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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. – villageidiot Apr 21 '12 at 3:30
I agree that it's not the best visual aid, but I think @RoryAlsop's argument is sill valid. – Hassan Apr 24 '12 at 0:15
I removed the deliberately over the top artists impression :-) – Rory Alsop Apr 24 '12 at 7:42
@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. – dmckee Apr 24 '12 at 17:48

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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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. – dmckee Feb 18 '14 at 1:22

## protected by Qmechanic♦Feb 18 '14 at 1:36

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