[mixmastamyk]

Interesting, I always thought of dark matter as just a lot of extra "dust", the dust that clumps to make the stars that make up the universe. The asteroid belt, the extra mass in our kuiper or oort clouds, etc. The dust that blocks parts of our view of the Milky way.

This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dark dust" not fit?

[BurningFrog]

The name isn't great. It sounds like matter that doesn't produce light, when it's really "matter" that's invisible.

But not matter you can touch. Which makes it not really matter in normal English. You probably can't see it either.

So "Invisible Untouchable Gravitation Sources" would be a more descriptive name. I don't think it'll catch on.

[bdamm]

Perhaps it could be some form of friction we don't yet understand? I mean a form of gravitational friction?

The nice part of being an armchair physicist is spouting off nonsense - I suppose if I really wanted to find out, I could make up a mathematical model of my theory and then find out if the evidence fits. Sadly, I really have no idea how to do that!

[themartorana]

Ditto. Out of curiosity, is dark matter spread unevenly throughout the universe, or is it directly inverse to matter?

Like, when we expect dark matter causes an effect, is it because we expect there to be a massive dark-matter "object" acting upon matter, or can we attribute it to denser dark matter as space between matter increases?

Edit: would viewing dark matter provide a negative image of the universe?

I'm asking mostly because I'm curious if there is dark matter, "light" matter, and empty space, or only one or the other? I love imagining this stuff, as if two different universes in a multiverse - ours and one we can't see - are acting upon each other and causing what we strive to explain. As if they were passing through each other.

[smeyer]

No, dark matter is definitely not the inverse of regular matter. Dark matter is spread unevenly through the universe, and it's spread in a pretty similar (but still notably different) way to the spread of regular matter. For example, where you have galaxies with lots of regular matter there is also lots of dark matter, but it has a slightly different distribution. You seem to be imagining something cool, but not something that bears much resemblance to our universe.

[themartorana]

Thanks for that! I realize it was a pretty n00b question, I appreciate you taking the time. :)

[bdamm]

So there could be dark matter stars, and dark matter planets? Dark-matter supernovas? Perhaps even in the same locations? Or is this strictly a galaxy-scale phenomenon and not an immediately local phenomenon?

[socceroos]

That's part of the problem with dark matter theory, isn't it? Theory predicts even and well distributed dark matter anti-particles, yet there isn't in the observable universe...

[smeyer]

I'm not sure what you're referring to. I don't think the theoretical problems around antimatter are any worse for dark matter than they are for baryonic matter.

[Ygg2]

A more interesting interpretation would be that the observable universe could be near a parallel unobservable universe and that gravity of that universe is influencing current universe.

[rootbear]

I agree that dark matter isn't the best name. We could call it ether...

[tjradcliffe]

There's nothing magical about the required properties, so I'm not sure why anyone would describe them that way. They are no different than the properties imputed to neutrinos or the aether: http://www.tjradcliffe.com/?p=1801

The important thing about extra-galactic dark matter is there is too much of it to be made of ordinary particles: protons and neutrons and electrons. We know how many ordinary nucleons there are because we have a pretty reasonable estimate of the primoridal ratio of hydrogen to helium. As the Big Bang cooled quarks eventually cooled down enough to make protons and neutrons. Free neutrons only live about fifteen minutes so they only had a short time to capture onto protons and make deuterium nuclei which could further colide to make helium. The ratio of He/H in the early universe is therefore a sensitive measure of the denisty of the universe at the time of nucleon condensation, and we know the radius from the temperature, so we can calculate the total number of particles. There aren't enough to make up extra-galactic dark matter. There are enough to make up galactic dark matter, so there may be two totally unrelated dark matter problems.

When physicists talk about "dark matter" we are sliding back and forth between meanings according to context, but this is all lost on outsiders, unfortunately. It's needlessly confusing, but unfortunately it's the way it is.

So amongst the perfectly ordinary, non-magical properties that extra-galactic dark matter has to have is to be something other then ordinary neutrons and protons. There are some other contraints on its various interaction strengths that come from the scale of galaxy formation and larger scale structures in the universe too. Again, these are perfectly ordinary properties, inferred based on the evidence in exactly the same way the properties of aether and neutrinos were.

Dark matter may or may not exist (neutrinos do, aether does not) but there is absolutely nothing out of the ordinary about it. It is just normal science doing what science normally does.

[vorg]

> Dark matter may or may not exist but there is absolutely nothing out of the ordinary about it

18% of matter is "regular" matter that interact with each other via bosons, and the other 82% is "dark" matter that doesn't interact with the known bosons. Could dark matter have its own "bosons"? Even more interesting, could dark matter be partitioned into various categories based on the "boson" they interact with? If so, perhaps the categories would have random but regularly distributed size ratios, for example:

* 25% of matter is zyzotic interacting via yetions

* 18% of matter is hadronic (i.e. quarks, leptons) interacting via bosons (i.e. photons, gluons, W, Z)

* 13% of matter is xenatic interacting via winnions

* 7% is vivacions interacting via ululons

* smaller partitions at 5%, 4%, 2%, 1.9%, 1.8%, 1.8%, 1.7%, 1.7%, 1.7%, 1.7%, etc etc etc

* there'd be millions of partitions overall

* the smallest partitions would consist of only 1 particle each (and perhaps its anti-particle) in the entire Universe

This would put various properties such as mass, etc of the hadronic particles as being randomly determined, perhaps quantumly, a split instance after the Big Bang.

[mixmastamyk]

I'll try to explain what I meant about magic. Your piece was interesting on how building mental models is an important part of science which opened my eyes a bit. However, in terms of this discussion I was thinking there might be lower-hanging fruit that makes dark matter appear farther-fetched.

For example, we have calculated the mass of a galaxy, but it doesn't spin right at that mass. So, instead of:

1. Assuming our calculations are wrong, or

2. There is more interstellar dust/gas/solar-wind mass than we realize, or

3. Heavier Black hole at center, or spinning speed/time dilation occurring?

4. Effects of gravity are weaker at very long distances

a different theory is presented instead.... that 2/3 of the universe is made of a brand-new undetectable substance.

As you mentioned it may or may not be true, and I've learned a lot about why it is convincing in this larger thread. Still, as Sagan said, "extraordinary claims require extraordinary evidence."

It's probably a flaw of the layman that ideas that have yet to be confirmed tend to appear magical.

[Dylan16807]

> 1. Assuming our calculations are wrong

That's what this is. We checked the math 20 times over and we know the model is wrong.

> 2. There is more interstellar dust/gas/solar-wind mass than we realize

We checked.

> 3. Heavier Black hole at center, or spinning speed/time dilation occurring?

Doesn't fit.

> 4. Effects of gravity are weaker at very long distances

Also an extraordinary claim, has not been ruled out.

Plus we already have confirmed 'undetectable' substances, so it's not very strange to suggest a new one.

[stan_rogers]

"Dark dust" would, in many circumstances, have at least an infrared signature (or, rather, a signature that originates in the infrared, even if it has shifted considerably downward by the time we see it). This stuff is "dark" in the sense that the only effects we can see at all (so far, at least) are gravitational. In terms of what we think we do know, that implies mass - but mass that doesn't interact with light except insofar as it distorts spacetime.

[mixmastamyk]

Interesting, does that mean there is not enough microwave or radio emission out there to signal a significant amount of dust/gas? The original article mentions this a bit, but I don't understand completely what he is saying since the graphs aren't given much explanation.

[deepsun]

Because dust would emit something, especially while being hot from nearby stars radiation. It would also absorb waves from background, to some degree. Dust and gasses are pretty familiar guys, comparing to dark matter.

[bashinator]

It's "dark", in that it doesn't interact with electromagnetic forces (photons and electrons). Neutrinos are one example of this kind of matter, so it seems reasonable that there are other kinds that are even more difficult to observe.

[aetherson]

There'd have to be so much of it that we'd see it. There's way more dark matter than normal matter in the universe (stipulating that dark matter theories as we currently understand them are at a coarse level correct).

If it were "dark dust," there would be enough of it that we would be able to detect it through means other than gravitational ones.

[mixmastamyk]

Thanks. Given that the Oort cloud is very sparse and we can't see it, and we can't (optically) see even the largest planets on other stars, why would we expect to be able to see it?

[teraflop]

The objects we know of are orders of magnitude too small to explain the missing mass. The Oort cloud is estimated to make up about 0.002% of the solar system's mass. If instead it was 5 times more massive than everything else put together (which would be required for it to make up our solar system's share of the galaxy's dark matter) it would be a lot more visible.

[dalke]

For what it's worth, https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla... is a list of the directly observed extrasolar planets.

[mtrpcic]

There's one other thing that goes against the idea of "dark dust". if it was "dark dust" that was just a sparse cloud of standard elements, we would be able to detect the makeup of the cloud by pointing a Spectrometer at some of the darker (but seemingly "denser") parts of the cloud to detect what it's made of.

[djokkataja]

In theory, dark matter is composed of particles that have mass (and can therefore produce gravitational effects that are noticeable if you have a bunch of them), but they do not interact with the particles that compose atoms via electromagnetic forces or via the "strong nuclear force" (the force that holds the nucleus of an atom together).

This means that dark matter is not like the "physical matter" that you're used to seeing. Dark matter doesn't absorb or emit light, so it doesn't block our view of some parts of the Milky Way. The stuff that blocks our view is basically "space dust", and that "space dust" is made of atoms, not dark matter.

According to the theories of dark matter, it can affect our view of other places in the universe, but only via "gravitational lensing". "Gravitational lensing" is where there are enough particles with lots of mass all together (like a galaxy) that they noticeably distort light as it goes by. There is compelling evidence for dark matter from observations of gravitational lensing effects near galaxies that do not have enough mass from their observable stars to account for the degree to which light is distorted as it goes past the galaxy--in other words, to produce that effect, some galaxies must have significant amounts of mass from particles which are not visible.

Why couldn't that extra mass be from "space dust"? Because there is an enormous amount of mass that we can detect via gravitational lensing, but which is not emitting or absorbing light. "Space dust" does not make up much mass for each galaxy compared to the stars and black holes. It's nowhere near the quantity required to produce the observed gravitational lensing effects.

Related: Space dust: https://en.m.wikipedia.org/wiki/Cosmic_dust

Dark matter & gravitational lensing: http://m.phys.org/news/2014-07-large-dark-peaks-gravitationa... - This article notes that dark matter is estimated to comprise about 80% of the mass of the universe. If dark matter was made of visible particles, that would be like having four times as much visible material floating around in galaxies as the stars and black holes which comprise the galaxies, and somehow we don't see any of it.

[InclinedPlane]

The current best explanation for all of the observational evidence is a "cold dark matter" (CDM) theory, where the dark matter is composed of "WIMPs" (weakly interacting massive particles). Neutrinos are WIMPs that we know exist, and compose some small fraction of dark matter, but neutrinos are "hot", they travel at relativistic speeds, rather than much slower speeds.

There have been many theories of what the "unseen mass" could have been made of before the CDM/WIMP theory gained ground, but those competing theories have been eliminated by observational evidence which contradicts their predicted effects. Large amounts of dust would block visible light and impose characteristic changes on the light that passes through it. We see dust in other galaxies and our own, but we don't not see nearly enough dust in the right locations and in the right quantities to account for the missing mass. So it can't be dust. Large gas clouds are another idea. Huge, transparent, whispy clouds of gas. But these too would affect the light passing through them, adding the spectral absorption signature of the gas to the light coming from the other side of the cloud. We can observe several large concentrations of gas around galaxies, and often the mass of that gas is larger than the total mass of stars in the galaxy, but it's still far too low, by an order of magnitude or more, to be the missing mass. We also see cases, such as the bullet cluster, where we can map the location of the stars, gas, and the overall mass (through gravitational lensing), and we see that the mass is not where the gas is. Another theory is that it could be not dust but bigger chunks of stuff, like planets or brown dwarfs, so called "MaCHOs" (Massive Compact Halo Objects). But to make up the missing mass there would have to be a great many of them. We can collect statistics on how common such objects are in our own galaxy and nearby galaxies using gravitational micro-lensing surveys. Because there are a huge number of stars in the sky, and if there were lots of MaCHOs floating around then every once in a while one would happen to be precisely lined up along the line of sight to another star and that arrangement would slightly brighten the remote star. We can monitor a huge number of stars using modern digital imaging systems and survey the MaCHO population this way. We've found several such micro-lensing events but again the observations put an upper limit on the MaCHO population which is far, far below what could possibly account for all of the missing mass.

No matter how you slice the observations, you still end up with a lot of missing mass and a lot of inconsistent observations. Unless you accept the possibility of WIMP dark matter. When you do that then all of the observations fall into place. Not just the above but also things like the large scale structure of the universe, the structure of the cosmic microwave background radiation, and so forth.

[pavel_lishin]

Nitpick: I don't believe neutrinos are classified as WIMPs, since they are either massless, or have an incredibly small mass.

[InclinedPlane]

Neutrinos conclusively have mass since they experience flavor oscillations (if they were massless that would be impossible). Current estimates are that the total mass of all flavors of neutrinos is a fraction of 1 eV (less than a billionth the mass of a proton).

Anyway, neutrinos aren't generally called "WIMPs" because they already have a name, the term "WIMP" is generally reserved for new particles that have yet to be directly observed.

[geon]

Does the "massive" in WIMP mean "have a mass" or "higher mass than other particles"?

[InclinedPlane]

It means it has non-zero mass. There are some theories of dark matter which would involve particles having thousands of times less mass than even a neutrino.

[xioxox]

The constraints from big bang nucleosynthesis (see e.g. http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto4.h...) say normal (baryonic) matter only makes up a small fraction of the mass in the universe. Therefore the remaining matter has to be non-baryonic (not normal gas or dust).

[Retra]

Like others said: dust simply isn't dark. At least not in the way implied in the term 'dark matter'.