Where's all the dark matter?

Factual Questions
1

Dark matter only interacts with normal matter through gravity. I understand that part. I imagine globs of dark matter being pulled toward big sources of gravity, but almost never settling in a stable way in any particular place (because after all, what force would ever stop it from moving? There can be no friction or anything to cause it to “clump” anywhere.)

So if my understanding of dark matter is more-or-less in line with what we actually know about it, dark matter would end up being everywhere, especially around things with mass. And the more mass, the more gravity, the more dark matter is near it. Over the eons, I would expect dark matter to be dispersed in kind of a chaotic manner among all the stuff in the universe, more or less evenly distributed around massive objects, but always a fluctuating amount of it.

Is this the case? Is there a bunch of dark matter occasionally throwing off our gravitational measurements of the earth, the sun, and the Milky Way? If not, why not? Why would dark matter be everywhere else, but not right here?

2

Most of the dark matter associated with the galaxy is in a spherical dark matter halo surrounding it.

Also, since the overwhelming majority of matter* is* dark matter, it’s not particularly attracted to “normal”* matter, but to itself. There’s just not enough normal matter to influence dark matter all that much; it’s (probably) the normal matter that has arranged itself around the dark matter, not the other way around.

  • Calling the minority of matter normal matter is a bit weird, I think.
3

It sounds like you’re asking “Is there dark matter right here, on and in the vicinity of Earth, and if so, why don’t we detect it?”. And the answer is, there presumably is, and we don’t detect it because it’s distributed more-or-less uniformly. Averaging over all of space, dark matter has a density several times higher than normal matter. But normal matter clumps. And where it clumps (like, say, here), it locally has a density that’s much, much greater than the uniform dark matter density. In a sphere, say, 100 AU in radius centered on the Sun (more or less what we can make good orbital measurements of in our Solar System), the amount of dark matter would be negligible compared to the mass of the Sun. But space is big, and if you have that much dark matter in every 100-AU sphere (most of which don’t have a star in them), it adds up.

4

Just a quick note … we detect dark matter at the “groups of galaxies” scale … yeah, this big even for astronomers …

5

We also detect it within individual galaxies. The galaxy-cluster results are mostly interesting in that they’re much harder to explain away with non-dark-matter models like MOND.

6

That’s the crux of my question. If there’s so much of it, why isn’t it constantly everywhere, including here? Why does it take a scale so large for us to notice it? Why isn’t our solar system made up of a ratio of 7|3 dark matter|regular matter?

7

Excellent question … there’s a mathemagical answer too, quite robust as I remember … I’ll leave it to the mathemagicians to explain … I’m dismissive of dark matter’s existence because I need more than one picture of it for proof … perhaps LIGO will provide the recipe I need for the boot leather I’ll be eating for saying such foolish things …

Hopefully someone will be along to explain … it’s really trick how it’s done …

8

It probably is here. We can only detect a change in the distribution in dark matter. Presumably the variation in dark matter in the region near us is too uniform to detect with current methods.

So it’s all relative. The absolute amount of dark matter is something else entirely.

9

It is here, in the same amount as it is everywhere. The Solar System doesn’t contain a 7:3 ratio of dark matter to baryonic matter, not because there’s less dark matter here than most places, but because there’s more baryonic matter. A lot more.

10

What I bothers me, is that there seems to be a bit of to-ing and fro-ing of the influence of dark matter, and some of this seems to be based upon some chicken and egg assumptions.

So, thinking aloud (and really want someone with a clue to correct me here), I guess the course of the universe sort of goes:

Very early universe - very hot, isotropic distribution of baryonic and dark matter.
Early universe - cooling down, inhomogeneities in distribution of all matter. Baryonic matter starts to clump very slightly. Gravity drives increasing inhomogeneity.
Genesis of clouds that form galactic clusters - formation of large regions of denser matter. Baryonic matter starts to clump more, and starts to differentiate itself in distribution from dark matter.
Individual proto-galaxies of matter form in a big spheres.

First stars. Baryonic matter clumps even more forming regions that allow a cascade of clumps to mutually attract and begin star formation. Dark matter does not clump and stays in diffuse spherical cloud for each each galaxy. Baryonic matter forms galactic disk within dark matter sphere.
Galaxies evolve, stars go through generations,

So, initially dark matter, along with baryonic drives the evolution of the layout of matter in the universe, but as baryonic matter clumps up, forms stars and galactic disks, the dark matter is left as a diffuse cloud.

I’m assuming lots of careful statistical mechanics has been done on this. But it would be illuminating to understand the assumptions and some of the actual results that have been derived. It seem that there is an implicit assumption that dark matter only interacts gravitationally - even with itself - as the assumption seems to be that it doesn’t clump. Or perhaps the numbers don’t work unless it doesn’t clump. (Which is what I mean by a chicken and egg question.)

But even as baryonic matter does form galactic disks, one would expect that the gravitational attraction of matter in these disks would drive some movement of dark matter and we would expect some increase in the density of dark matter in the galactic disks, and perhaps some increase near stars. One assumes the numbers have been crunched - but again, knowing what they are would be nice.

There seems to be a tantalising detective mystery here. One that has not been elucidated to us mere mortals.

11

The numbers don’t work unless we assume that most, at least, of the dark matter doesn’t interact non-gravitationally, and hence doesn’t clump. But it’s still possible that some of it interacts in some other way, possibly some way that we have never observed in known forms of matter, in which case we would, in fact, find a higher concentration of dark matter in the disk. This is a real possibility that some physicists are actually taking seriously, and for which there is even a small amount of (very weak) evidence. And it turns out that an upcoming space mission will be able to resolve the question, even though that’s not what it was designed for specifically.

12

I believe that dark matter really is clumped in galaxies. However, it isn’t particularly clumped in solar systems.

And this is because the clumping mechanisms in galaxies and solar systems are different. Galaxies are bound together almost entirely by gravity. And that is, we theorize, the only force that affects dark matter. Stars and planets orbiting the galaxy almost never interact, despite billions of them whirling around chaotically. They almost never collide or become gravitationally bound to each other, instead they just keep on spinning.

Solar systems are drawn together by gravity as well. You have a cloud of gas, it contracts due to gravity, and you eventually get a star or multiple stars, and various other smaller clumps. But if you imagine the trajectories of individual atoms in the cloud, it’s clear that electromagnetic interaction is very important in forming dense clumps. An object in motion stays in motion unless acted upon by an outside force.

So imagine a hydrogen atom sitting there. It starts to fall towards the center of the nebula. If it has no other interactions, it will fall towards the center, and then continue back out of the center with the exact same energy. It will either form an elliptical orbit if it has less than escape velocity, or a hyperbolic orbit if it has greater than escape velocity. However, that hydrogen atom probably isn’t going to have no other interactions. It will have electromagnetic interactions with other infalling hydrogen atoms…they will bump into each other and slow each other down. So lots of atoms are going to fall into the center, and instead of falling back out of the center, they bump into another atom falling in from the opposite direction, and both atoms stop and are left in the center. And as this happens to more and more atoms, the denser and denser cloud is more and more likely to electromagnetically interact with even more infalling particles and you have a snowballing accumulation into a star, or planet.

Compare that to a hypothetical dark particle. It falls into the center, right past all the other infalling baryonic particles and other dark particles, and just waves hi to them and doesn’t interact with them in any way. And so that dark particle continues on in its elliptical or hyperbolic orbit, never slowing down. You will never have a central concentration of dark particles like you have with baryonic particles.

This is all theorizing that dark particles really don’t interact in any way with baryonic particles, or with each other, except by gravity. Maybe there is some subtler interactions that we having taken into account. However, the reason we don’t believe in them is that we don’t have any evidence for this. The only evidence we have for dark matter is that there seems to be a lot more gravity than there should be, and the simplest explanation for this are various flavors of weakly interacting massive particles.

The reason we can’t just reach out and put these little dudes under a microscope and study them directly is just because they don’t interact electromagnetically or by nuclear forces, only by gravity, so how exactly are you going to detect them?

Sure, there are probably millions of them streaming through your detector every second, but if they don’t interact with your detector, you don’t detect them. And there aren’t as many dark particles on Earth as there are baryonic particles, because if there were Earth’s gravity would be twice as strong.

So dark matter isn’t concentrated in planets or stars or nebula. It is only concentrated in galaxies. There isn’t any more dark matter in our solar system than there is in a random solar-system-sized chunk of interstellar space in our galaxy. So even though there seems to be more dark matter than normal matter in the universe, there isn’t more dark matter than normal matter in our solar system.

13

But the original formation of galaxies and, in particular stars, was driven entirely by gravity. So if dark matter interacts gravitationally, and since there is much more of it, why didn’t it clump into galaxies and stars even if those stars didn’t start to shine. AFAIK, even now, it is gravity and only gravity that keeps the sun in one piece.

14

Yeah, but it wasn’t just gravity that got the mass into the star in the first place. You only need gravity to balance the nuclear reactions occurring - and it is the nuclear reaction that stops the star collapsing rather then the other way around.

Baryonic matter interacts with the electromagnetic force. Atoms form, molecules from them, clumps of molecules form due to differential electrostatic charges - van der Waals forces etc etc. Eventually the odd clump forms that starts to have enough mass to attract other large clumps gravitationally. Absent the EM force you would still have a diffuse gas of matter. Whenever two particles came close they would either wizz past one another, or bounce off one another. Gravity is too weak to make things stick until the relative speeds are very very low. Much lower than the speeds EM forces can overcome to make things cluster.

This seems to be the really interesting point about dark matter. The basic numbers require that there is no - or only a very weak - mechanism to make it clump in the manner EM does for baryonic matter. I’m pretty impressed that the numbers could be crunched enough - with enough observational data to constrain the calculations - to show this.

15

The formation of galaxies was driven almost entirely by gravity, but not stars.

Think about it. If you have a cloud of gas in space, it has mass, and the individual atoms in the cloud are attracted to the center of mass of the cloud. So they start falling towards the center.

But unless some force acts on them, they’re going to fall back out of the center with the exact same velocity as they had when they fell in. Just like, say, Haley’s Comet. It falls toward the Sun because it’s attracted by the Sun’s gravity, but when it gets near the Sun it’s going really fast and then it flies back out into the outer solar system again.

This is how particles of dark matter in a primordial nebula would behave. Sure, they’re affected by gravity. But they don’t stop when they reach the center, because for them to stop some force would have to act on them, and the only force acting on them is gravity. So then fall towards the center, and then get thrown out again, in either an elliptical orbit, or in a hyperbolic orbit where they’re flung out of the nebula entirely.

The difference between dark matter particles are baryonic matter is that the baryonic matter is affected by gravity, but it’s also affected by other forces. So imagine that comet falling towards the Sun. If no forces act on it, it will go around the Sun and back out into the outer solar system. But if it gets close to the Sun, close enough to hit it, it will be affected by electromagnetic forces. It will be slowed by friction. It will fall into the Sun and stay there, just like a thrown baseball doesn’t keep on going, it is stopped by electromagnetic interaction with the ground. A bunch of dark matter particles wouldn’t act like the baseball, they would just fall right through the ground as if it weren’t there, and orbit around inside the Earth forever.

So in the formation of stars, the hydrogen atoms are attracted to each other by gravity, but they only form clumps because friction with each other stops them from falling back out of the forming star.

Yes, if you magically turned off gravity in the star, the atoms in the star would be repulsed from each other by electromagnetic forces, and the star would explode. But the star itself couldn’t have formed without electromagnetic forces, it would have stayed an extremely diffuse cloud of orbiting weakly interacting particles.

16

How do we know that dark matter does not interact via the nuclear forces particularly the weak force. Those are very short range forces. For them to apply, doesn’t matter have to clump first into stars so the particles are close enough to interact via the nuclear forces.

This seems particularly true of the weak force. Neutrino’s only interact gravitationally and via the weak force. Couldn’t something like a massive neutrino (and therefore not moving fast) be cold dark matter?

17

Dark matter certainly could interact via the weak force, and the default assumption is that it probably does, but you’re not going to get very much clumping from that.

18

The answer to the OP: “You’re soaking in it, Madge.”

As for weak force interactions, that’s what the various attempts at discovering it are counting on. After all, that’s the only way to detect neutrinos (which are technically one form of dark matter, but they’re very low mass and can’t make up more than a small fraction of dark matter.) So far, no luck for any of them, but that doesn’t stop physicists from trying.

19

Thanks, that was exactly the point of my question, but I also wondered if it could react via the strong force Would the strong force be enough for dark matter to clump? I’d think it would have to clump before particles were close enough to react via the strong force.

20

Well, we can’t completely rule it out until we figure out what the stuff is, but it probably doesn’t interact via the strong force. So far as we can tell, the strong force acts on all things that contain quarks and only those things, and all quarks have a charge, so anything that’s subject to the strong force is also subject to electromagnetism.

Now, maybe there’s some hitherto-unknown quark flavor that doesn’t have a charge, or a non-quark particle that interacts strongly, but that’s not the way to bet.