# Forum > Discussion > Mad Science and Grumpy Technology >  Why is dark matter presumed to all be a single type of thing?

## Bohandas

I've heard a lot of different things on different educational programs about why dark matter couldn't actually be various mundane things, and usually the explanation given is that someone already checked about the thing in question and found that it could only account for some small percentage of the extra mass that seems to be there. Usually the percantage is less than 50% but non-negligible, something in the 0.5%-10% range.

I've heard this about so many different things that I'm starting to wonder if anyone's ever actually taken the time to do the commonsense next step and add all these results together. There are enough of them that it feels like together they should add up to something significant.

Has this calculation been done? And if so, why isn't it talked about more?

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## NichG

> I've heard a lot of different things on different educational programs about why dark matter couldn't actually be various mundane things, and usually the explanation given is that someone already checked about the thing in question and found that it could only account for some small percentage of the extra mass that seems to be there. Usually the percantage is less than 50% but non-negligible, something in the 0.5%-10% range.
> 
> I've heard this about so many different things that I'm starting to wonder if anyone's ever actually taken the time to do the commonsense next step and add all these results together. There are enough of them that it feels like together they should add up to something significant.
> 
> Has this calculation been done? And if so, why isn't it talked about more?


A lot of the candidates are theorized particles, not things which can be observed and have their mass budget measured. So it wouldn't be correct to add those up, since we don't even know for each if it exists or not. A randomly grabbed survey paper of candidates lists five, only one of which is known to exist (https://arxiv.org/pdf/0903.4849.pdf).

So there may not actually be so many mundane candidates that actually satisfy the requirements (sufficiently weak interaction with other matter, distributed in the right way to explain galaxy rotation curves,etc). That paper only lists neutrinos as such a candidate, and the bound on their contribution is about 10%.

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## Razade

Isn't the answer to this question pretty easy in that: It isn't. Why do you assume that that's the presumed answer, as if we have a presumed answer at all?

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## Cikomyr2

Dark Matter is a catch-all term to account for the mass of the universe that isn't perceivable via electromagnetic mean(i.e. "dark"), but are still included in the mathematical equation meant to understand the movement of the universe; i.e. it has mass ("matter").

Now, this can be one thing (unlikely) or a billion, or billion of billion of billions of different things. Until we identify these things, we will keep calling them "dark matter" unless we somehow update the math meant to represent our understanding of the universe to not need this missing mass.

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## Mechalich

> Dark Matter is a catch-all term to account for the mass of the universe that isn't perceivable via electromagnetic mean(i.e. "dark"), but are still included in the mathematical equation meant to understand the movement of the universe; i.e. it has mass ("matter").
> 
> Now, this can be one thing (unlikely) or a billion, or billion of billion of billions of different things. Until we identify these things, we will keep calling them "dark matter" unless we somehow update the math meant to represent our understanding of the universe to not need this missing mass.


It could quite easily be _mostly_ one thing. Visible matter is basically two things: Hydrogen (75%) and Helium (25%), with all the rest being a minute fraction of the total. It wouldn't be surprising that whatever accounts for dark matter has a similar distribution setup.

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## Bohandas

> So there may not actually be so many mundane candidates that actually satisfy the requirements (sufficiently weak interaction with other matter, distributed in the right way to explain galaxy rotation curves,etc). That paper only lists neutrinos as such a candidate, and the bound on their contribution is about 10%.


Right. 

Similarly I've also seen an educational video that talked about how massive compact halo objects (ie, black holes, black dwarves, rogue planets) were ruled out because the upper bound for them based on microlensing data was around u6only 20% of the mass of dark matter.

What I'm saying is that that makes it look, to me, like the two of them _together_ could still potentially account for up to 30% of this missing mass. And I'm pretty sure there's other potential mundane sources. It seems like together they could potentially account for all of it (and even if not all of it, then still potentially at least enough of it that it might change the character of what you would expect any new particles to look like)

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## NichG

> Right. 
> 
> Similarly I've also seen an educational video that talked about how massive compact halo objects (ie, black holes, black dwarves, rogue planets) were ruled out because the upper bound for them based on microlensing data was around u6only 20% of the mass of dark matter.
> 
> What I'm saying is that that makes it look, to me, like the two of them _together_ could still potentially account for up to 30% of this missing mass. And I'm pretty sure there's other potential mundane sources. It seems like together they could potentially account for all of it (and even if not all of it, then still potentially at least enough of it that it might change the character of what you would expect any new particles to look like)


I'd just be careful, because I think there are multiple constraints other than 'can't see it' and 'mass budget' which rule out a lot of these. There are weird bounds on what the equation of state of dark matter can look like in order to be consistent with rotation curves and simultaneously with the role it plays in the overall dynamics of expansion. If you don't hear people talking about a particular candidate anymore it may well be because one of those other constraints excluded it more strongly.

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## BloodSquirrel

> I've heard a lot of different things on different educational programs about why dark matter couldn't actually be various mundane things, and usually the explanation given is that someone already checked about the thing in question and found that it could only account for some small percentage of the extra mass that seems to be there. Usually the percantage is less than 50% but non-negligible, something in the 0.5%-10% range.
> 
> I've heard this about so many different things that I'm starting to wonder if anyone's ever actually taken the time to do the commonsense next step and add all these results together. There are enough of them that it feels like together they should add up to something significant.


The problem with this, conceptually, is that it assumes a systematic error towards minimizing the mass of the universe when you have no a priori reason to expect that all of your errors in measurement should flow in one direction.

If your models are wrong, you would expect some of them to underestimate the mass of the universe, and some of them to overestimate, and fixing a bunch of small things in your model, rather than one big one, should result in a bunch of things cancelling each other out. Now, there are reasons why this might not be the case- one fix might have an effect several orders of magnitude above the others, or there is an overarching assumption built into your model which systematically resulted in errors that minimize mass rather than maximizing it- but that still leaves you looking for what that one reason actually is.

"We assumed the lower bound in every case when estimating the mass of universe" would be one such reason, but I doubt that they've done that.

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## Cikomyr2

> It could quite easily be _mostly_ one thing. Visible matter is basically two things: Hydrogen (75%) and Helium (25%), with all the rest being a minute fraction of the total. It wouldn't be surprising that whatever accounts for dark matter has a similar distribution setup.


I am not going to disagree with that. Some other posters have explained that the math may actually have yielded some answers.

The point I wanted to convey is that Dark Matter is not a thing. It's merely a "Gap" that our understanding of the universe has highlighted. Bull**** like "oh this thing is created OUT OF DARK MATTER" in some pop sci-fi stories just indicate that some sci-fi writers have no understanding of the science behind the formulation of what Dark Matter is.

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## Willie the Duck

> The point I wanted to convey is that Dark Matter is not a thing. It's merely a "Gap" that our understanding of the universe has highlighted. Bull**** like "oh this thing is created OUT OF DARK MATTER" in some pop sci-fi stories just indicate that some sci-fi writers have no understanding of the science behind the formulation of what Dark Matter is.


Or they are conjecturing the explanation for the solution to the gap being an undiscovered form of particles which can conveniently form into things, since that serves the needs of their storytelling.

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## Quizatzhaderac

One of the things is that the max of the range offered is plausible, not likely.

Wikipedia says there are enough MACHOs "to make up perhaps 20% of the dark matter in the galaxy". Let's interpret that statement to mean there's a 95% chance that dark matter makes up less than 20% of the dark matter in the galaxy. 

Then suppose you have five similar "up to 20%" sources. It's a 5% chance for each to be at/above the max. For all five it's a 0.05^5 = 3.125*10^-7 = 1 in a 30 million chance.

For some of the more exotic stuff, the kind that we're not sure exists at all, if it can't explain dark matter, we're less sure it exists at all.


> The point I wanted to convey is that Dark Matter is not a thing. It's merely a "Gap" that our understanding of the universe has highlighted. Bull**** like "oh this thing is created OUT OF DARK MATTER" in some pop sci-fi stories just indicate that some sci-fi writers have no understanding of the science behind the formulation of what Dark Matter is.


Yes and no.

That is definitely the original definition. However, it seems pretty common (even in technical contexts) to use the phrase "dark matter" to mean the non-baryonic matter contributing to the first definition.

We're also describing dark matter candidates as dark matter. I won't be the least bit surprised if we confirm dark matter candidates and keep calling them dark matter. After all, we didn't rename "atoms" after we discovered they were composite.

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## gomipile

> We're also describing dark matter candidates as dark matter. I won't be the least bit surprised if we confirm dark matter candidates and keep calling them dark matter. After all, we didn't rename "atoms" after we discovered they were composite.


To be fair to future scientists, if a confirmed candidate continues to be undetectable directly in the electromagnetic spectrum at galactic+ distances, it'll still be "dark" in the same way it is now.

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## gbaji

> I am not going to disagree with that. Some other posters have explained that the math may actually have yielded some answers.
> 
> The point I wanted to convey is that Dark Matter is not a thing. It's merely a "Gap" that our understanding of the universe has highlighted. Bull**** like "oh this thing is created OUT OF DARK MATTER" in some pop sci-fi stories just indicate that some sci-fi writers have no understanding of the science behind the formulation of what Dark Matter is.


That's how scientific models work though. We don't have to know the specifics of what something is, only the parameters it needs to fill. As we learn more, we fill in more of the model. Yeah. I agree though that sci-fi writers sometimes (often?) take this and run with it in ridiculous ways.

The point of dark matter as a model is that it's "something" that does not appear to interact on the electromagnetic range (at least that we can detect), but that has "mass". Or specifically that there is some additional gravitational force requred (and yeah, I know that's not technically correct terminology either) to explain large scale movments of objects we can observe in the universe around us. Something with mass that doesn't show up as anything other than gravitational effect is the closest we can map to at this point. It's also something we can't detect locally either, which adds to the mystery.

It could literally be some particle that is EM neutral, but has mass, and has some intrinsice repulsive force (which would have to be fairly strong actually) between itself and other particles of the same type (which seems to be a trend anyway). This would cause said particle to spread out relatively evenly througout the universe, only concentrating where gravity wells are strong enough to do so (but only somewhat, otherwise we have other problems). Um. There's still a ton of problems with such a simple model though. Make this make up 90% of the actual mass of the universe, and then do a ton of additional math that I'm not qualified to do, and maybe you're somewhere on the right track. At least in terms of effects we can percieve.

And that's going in the Occam's Razor "least complex/new explanation". We could also speculate other things that could explain the observed spin effects as well. What we're precieving is at large scales (like galactic size scale), not small or medium, or even "somewhat large". It's also entirely possible that we could be seeing something completely new/different and outside of our frame of reference. Some new force that just doesn't act on a scale smaller than that maybe?

The example I like to use for this is to imagine a race of tiny beings who evolved intelligence while living on the surface of a refrigerator magnet. Everything they know operates off the rules of magnetism and that is the "strongest" force within their frame of reference. Now, imagine they build powerful telescopes at some point to observe the larger vast universe (of the kitchen I guess?). And while observing things, they notice that these vastly large objects move in a way that magnetism just can't explain. Heck. Everything on that larger scale does. They move in a direction and way that they don't see as "down" from their perspective at all. Of course, on that large scale, gravity is more powerful than magnetism, so objects moving towards the floor of the kitchen instead of towards the fridge is perfectly explainable. But not to our hypothetical fridge magnet people.

So... Could be something like this. Who knows? But yeah. We always start models out by using what we know to try to explain what we see.

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## johannessmid

> I've heard a lot of different things on different educational programs about why dark matter couldn't actually be various mundane things, and usually the explanation given is that someone already checked about the thing in question and found that it could only account for some small percentage of the extra mass that seems to be there. Usually the percantage is less than 50% but non-negligible, something in the 0.5%-10% range.
> 
> I've heard this about so many different things that I'm starting to wonder if anyone's ever actually taken the time to do the commonsense next step and add all these results together. There are enough of them that it feels like together they should add up to something significant.
> 
> Has this calculation been done? And if so, why isn't it talked about more?


Yes of course people have done this.  A lot of work in the 90's went into modeling different mixes of hot+ warm + cold dark matter for example.  Then people start looking into what would fill the cold dark matter slot or the hot dark matter slot.  "Hot", "warm" and "cold" refer to the characteristics of the dark matter - many different proposed particles might fit a given slot.  It's not necessarily useful at this point to consider "cold dark matter is 50% A, 40% B, and 10% C" is we haven't discovered any A, B or C but they all have similar observable effects.  Once we discover A it will be interesting to see if the observed mass, cross-section, detection rate etc. seem to describe all of the observable effects, or if there is not enough A to match what we see out in the universe.  That's when it will get really interesting.

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