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What is the estimated density of the dark matter? Some clumps of dark matter might have higher density and others might have lower density. So what is the estimated density range of dark matter?

Is it possible for clumps dense enough like for example iron to exist? If you get close to it, can you grab a piece and put it in a bucket? Can you make invisible objects from it? Say for example a ball or a hammer made of dark matter.

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    $\begingroup$ xkcd.com/2186 $\endgroup$
    – John Doty
    Feb 17 at 15:26
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    $\begingroup$ @JohnDoty I sometimes wonder why we need Stack Exchange when we have XKCD. $\endgroup$
    – Barmar
    Feb 17 at 15:31

3 Answers 3

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The average density of dark matter over the entire universe is estimated to be $2.2 \times 10^{-27}$ kg/m$^3$ (sometimes quoted in the odd units of $1.2 \times 10^{-6}$ GeV/cm$^3$ or the more astrophysical $3.2\times 10^{-8}$ $M_\odot$/pc$^3$). This is uncertain to the tune of about 10% (in the context of the prevailing cosmological model) due to uncertainty in the current value of the Hubble parameter.

However, even though dark matter (by definition) does not interact electromagnetically it is still affected by gravity and this does allow it to "clump" simply via gravity (e.g. see here) and achieve much greater concentrations in deep gravitational potential wells (e.g. at the centres of large galaxies like the Milky Way).

For example Sofue (2020) estimate a dark matter density of 840 Gev/cm$^3$ at the centre of the Milky Way, falling to about 0.3 GeV/cm$^3$ at the location of the Sun. Similar peak densities appear to be reached in the centres of dwarf galaxies (e.g. Chang & Necib 2021). Meanwhile cosmic voids are thought to have a density a factor of 10 lower than the mean density of the universe.

There is thus something like at least a 9 order of magnitude variation in the dark matter density between the densest and least dense places in the universe, but even at its densest it is many, many orders of magnitude less dense than air for example. Note that since dark matter is thought to be non-interacting there is nothing to stick it together non-gravitationally; therefore it is unlikely to be in the form of anything but a smooth particle distribution, except perhaps if it were in the form of primordial black holes.

Dark matter appears not to clump on scales smaller than dwarf galaxies. In particular - there is no sign of it in star clusters. So there are unlikely to be small dark matter structures.

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  • $\begingroup$ Per the other answer - maybe not say "clump" because that implies inter-particle bonding, as opposed to increased density the way an ideal gas has higher density in higher gravitational fields. $\endgroup$ Feb 16 at 13:54
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    $\begingroup$ @CarlWitthoft Dark matter clumps It has over densities of 8 orders of magnitude. The clumps are large and have nothing to do with interparticle interactions besides gravity. $\endgroup$
    – ProfRob
    Feb 16 at 14:48
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    $\begingroup$ While true, it's misleading that the average density is lower than air: a cubic meter of space that happens to contain a hammer also has a lower average density than air. $\endgroup$
    – fectin
    Feb 16 at 23:12
  • $\begingroup$ My point is that the question was about whether dark matter can be clumped up into handheld objects. Your response on air density is exactly right: depending on the scale you look at, density may be much more or less homogenous. So, what information convinces you that dark matter is basically homogenous on the scale of handheld objects? edit: sorry, this was to your now-deleted comment $\endgroup$
    – fectin
    Feb 16 at 23:49
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    $\begingroup$ I have no idea by what mechanism dark matter fails to interact electromagnetically, so I have even less idea what other force might affect it :) Nonetheless, I think that edit is was the missing piece. It certainly convinces me. $\endgroup$
    – fectin
    Feb 16 at 23:55
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The density of dark matter is very low. 0.000000000000000000001 kg per cubic metre (source1 source2). And it like a fluid. It doesn't interact with light and it doesn't interact with itself (except by gravity) so it forms a vast halo, much larger than the galaxy. It is more dense near the centre of the galaxy, by a factor of maybe 100 or more. That's still not very dense by human standards

As dark matter doesn't interact with itself, it can't form structures. It can't be constrained to a bucket (it doesn't interact with the atoms in a bucket so would just leak away) You can't make a ball or hammer, as there is nothing to keep the dark matter particles bonded to each other. Balls are only possible because atoms can form strong chemical bonds.

There is the caveat that we don't know what dark matter is. So perhaps there is some room for doubt. We do know, from microlensing observations, that there aren't planet or star-sized clumps of dark matter.

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    $\begingroup$ "like a fluid" --> more like a gas than a liquid, in the sense that it "fills the container" , right? Tho' I agree that right now , dark matter is simply a BlackBox that we have yet to parametrize or describe. $\endgroup$ Feb 16 at 13:52
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    $\begingroup$ It would flow right through the container. If its a gas, its a zero pressure gas, or so close to zero it makes no difference: physics.stackexchange.com/questions/422503/dark-matter-pressure $\endgroup$
    – James K
    Feb 16 at 14:56
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Is it possible for clumps dense enough like for example iron to exist?

We don't know.

Maybe, at some point in the future when the dark matter finds itself cold enough to enable some kind of interaction within itself, it may condense, crystalize or clump in dense enough structures.

If you get close to it, can you grab a piece and put it in a bucket?

Not in the common understanding of "grabbing". Our everyday interactions with objects around us are without exception electromagnetic. Well, Earth gravity is a thing, too. But it is electromagnetic interactions that keep you from sinking in the floor.

Dark matter is known exactly for its inability to interact electromagnetically. This is why your hand, as well as your bucket will let any amount of this "substance" to pass through.

Can you make invisible objects from it? Say for example a ball or a hammer made of dark matter.

Make? You will have to interact with it in order to "make" something out of it. It is not only invisible, it is untouchable as well.

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