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Some people speculate that the mysterious dark matter in the universe could be tiny black holes. But on the other side, could dark matter particles attract each other by gravity and finally form a black hole? Since dark matter is even more abundant than normal matter, the dark matter black hole should not be rare.

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2 Answers 2

I think the problem with matter that only interacts gravitationally is that it's hard to get it all to stay in one place. Nebula slowly form stars and planets in part because of collisions between particles lead to larger particles, which tend to attract further particles. But particles that just wizz right through each-other can't coalesce without violating conservation of angular momentum. That's not to say that it's impossible, however. Just that the dynamics are different.

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You are right. If dark matter only interact gravitationally, then it's hard for them to accumulate. For example, considering two dark particles, when they "collide"( which means overlapping in space time), they pass through each other. If more particles are added in, a chaotic soup may form. This is much easier to simulate in a computer program. However, whether dark matter really doesn't interact through other forces is not clear. –  Fine Observer Dec 20 '13 at 21:32
    
@FineObserver In fact this is what we observe in gravity-only simulations. When galaxies collide, so do their dark matter halos, but they mostly passes right through each other. This forms large dark matter filaments through and around galaxies. –  Kevin Driscoll Dec 20 '13 at 21:41
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@KevinDriscoll I think you should maybe expand that comment into an answer as it describes explicit simulations that exactly answer the OP's question. Lionel's answer is good, but you're clearly an expert thinking exactly about the OP's question in your day job: it would be great to get such an authoritative answer on this site. Too many of us are generalists and, whilst some of us have excellent physical intuition and give great answers, it's not quite the same. –  WetSavannaAnimal aka Rod Vance Dec 20 '13 at 21:51
    
Here's a synopsis of a Physics review letter on the topic ;-) –  Waffle's Crazy Peanut Dec 21 '13 at 10:10

Dark Matter has not been detected within the vicinity of Earth by even extremely sensitive detectors such as the LUX (Large Underground Xenon) detector. Yet scientists believe that the halos surrounding galaxies are replete with Dark Matter.

It seems more likely that the LUX detectors could not detect Dark Matter simply because there isn't any in the vicinity of our Earth. That would essentially mean that Dark Matter distribution is perhaps non uniform across our galaxy. Since it seems to be localized in galactic halos, and perhaps such regions that are beyond the gravitational field of massive bodies (such as planets/stars/galaxies). Therefore it appears that Dark Matter could be gravitationally repulsive with respect to normal matter.

In that case, it makes perfect sense why Dark Matter cannot form Black Holes since they simply cannot be packed close enough (due to their 'soft' mutual repulsion), to form Black Holes. That also would explain why they are found mostly in the outer reaches of galaxies, within their halos.

For more details please see http://urbanyogi2012.blogspot.ca/2014/06/on-dark-matter.html

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"It seems more likely that the LUX detectors could not detect Dark Matter simply because there isn't any in the vicinity of our Earth." - I disagree. The ability of LUX and other experiments to detect DM depends on how stongly DM interacts/decays etc. There is no strong reason to conclude that there are no DM close to Earth - just that it does not interact/decay stong enough to allow for a detection. –  Winther Jun 20 at 1:31
    
"Therefore it appears that Dark Matter could be gravitationally repulsive with respect to normal matter." - This would be very hard to recogncile with our very successful models of structure formation - which treats DM as a purely collisionless fluid - and which interacts with gravity just as normal matter. –  Winther Jun 20 at 1:34
    
The 'strong reason' is they haven't been detected despite such powerful detectors! Dark Matter is supposed to be more than 5 times as abundant as normal matter, yet not a trace has been detected in the vicinity of Earth. Simple logic dictates that they have not been detected simply because DM is not present on Earth. –  harismind Jun 20 at 1:52
    
Your 'successful models' don't explain anything about DM. If it gravitationally attracts and is 5 times more abundant than normal matter then you should be seeing DM everywhere. You don't. You only see it in Galactic halos far removed from the gravitational ambit large bodies. Ask yourself why this pattern is observed consistently from galaxy to galaxy. –  harismind Jun 20 at 1:59
    
Firstly, we cannot "see" things that don't emmit light. The effect we "see" is though its gravitational effect. Right, such models do not explain what DM really is, but they tell us a lot on how it must act and we can say with great certainty that DM is not replusive as you suggest. That would have huge observable consequences on many observables such as 1) the cosmic microwave background 2) clustering statistics of galaxies and 3) galaxy rotation curves to mention some for which we have high quality observations of. –  Winther Jun 20 at 2:20

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