Is dark matter hiding in plain sight?

In summary, the Coasting Cosmology model, which does not require inflation, is more consistent with the observation of a higher baryon density than in the \LambdaCDM model. There could be a variety of Dark Matter, including quiescent Black Holes, IMBHs, or including much smaller and up to intermediate mass Primordial Black Holes.
  • #1
mintparasol
78
0
Hi all,
Apologies if my understanding is a bit simplistic but even if my question turns out to be idiotic, I'm sure I'll learn from your replies. Thanks in advance!


So the reason dark matter was postulated in the first place is because the observed gravitational effects in the known universe far exceed the observable matter in the universe, right?
But we're pretty sure there's a supermassive black hole at the centre of our galaxy and of most other galaxies and that our galaxy probably contains a large amount of smaller black holes, right?
So we've never directly observed a black hole. Is it not then possible that this 'extra' matter that we can't see exists in the form of black holes and not dark matter?
 
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  • #2
The short answer is that it is very unlikely that black holes constitute a significant portion of dark matter. Observations of Baryon Acoustic Oscillations and the observed chemical abundances show that there needs to be a large amount of non-baryonic matter in the universe.
 
  • #3
mintparasol said:
So the reason dark matter was postulated in the first place is because the observed gravitational effects in the known universe far exceed the observable matter in the universe, right?
No, it is because the observed gravitational effects within the discs of galaxies is not a simple curve as it should be. There must be some extra mass that is unaccounted for distributed throughout the galaxy disc.

But that doesn't refute your idea, so:

The thing about black holes is that they are very visible. They are surrounded by an accretion disc of infalling matter that radiates characteristically in the X-ray band. We don't see this, thus no unaccounted-for BHs.
 
  • #4
DaveC426913 said:
No, it is because the observed gravitational effects within the discs of galaxies is not a simple curve as it should be. There must be some extra mass that is unaccounted for distributed throughout the galaxy disc.

But that doesn't refute your idea, so:

The thing about black holes is that they are very visible. They are surrounded by an accretion disc of infalling matter that radiates characteristically in the X-ray band. We don't see this, thus no unaccounted-for BHs.

Hmm, I wasn't saying they were not accounted for, just that maybe there's more mass within them than they're given credit for...
And, no, actually, there could be countless black holes unaccounted for. Can you show me a picture of an accretion disc?
 
  • #5
nicksauce said:
The short answer is that it is very unlikely that black holes constitute a significant portion of dark matter. Observations of Baryon Acoustic Oscillations and the observed chemical abundances show that there needs to be a large amount of non-baryonic matter in the universe.

But how can we talk about chemical abundances in relation to black holes? Surely matter as we know it ceases to exist within the event horizon of a black hole?
 
  • #6
mintparasol said:
And, no, actually, there could be countless black holes unaccounted for. Can you show me a picture of an accretion disc?

Not so much pictures of accretion dics, but observational evidence of black holes:
http://apod.nasa.gov/apod/ap951230.html
http://apod.nasa.gov/apod/ap970516.html
http://apod.nasa.gov/apod/ap050128.html
My point was not simply that accretion discs alone are the signature of BHs, but that BHs - unlike what most laypeople seem to assume - are not undetectable. They are quite detectable.
 
  • #7
mintparasol said:
But how can we talk about chemical abundances in relation to black holes? Surely matter as we know it ceases to exist within the event horizon of a black hole?

The abundances of various light elements depend strongly on [itex]\Omega_B[/itex]. See the first figure here http://www.talkorigins.org/faqs/astronomy/bigbang.html#lightelements. If dark matter actually was baryonic (i.e., black holes) and [itex]\Omega_B[/itex] was 10 or so times bigger, then we would expect to see about 10 times more Li-7 than we do, and around 10 times less Deuterium than we do (as you can read off of that graph). Unless by some unknown process Li-7 preferentially falls into black holes compared to other elements, and by some other unknown process deuterium is created in large amounts, the situation is impossible.
 
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  • #8
You may find the following eprint paper interesting Nucleosynthesis in a Simmering Universe , which was published in a peer reviewed journal as NUCLEOSYNTHESIS IN A UNIVERSE WITH A LINEARLY EVOLVING SCALE FACTOR.

The Coasting Cosmology model, (R(t) = t) does not have a horizon or flatness problem, (so the model does not require Inflation), in addition the baryon density is much higher than in the [itex]\Lambda[/itex]CDM model, and so could accommodate all Dark Matter.

This DM might be in the form of quiescent Black Holes, IMBHs perhaps as the remnant of an epoch of Pop III stars, or including much smaller and up to intermediate mass Primordial Black Holes.

In order to obtain a linear expansion factor the DE would have to have an equation of state [itex]\omega = -\frac{1}{3}[/itex], dominated by a form of DE called 'K Matter' by Kolb in A coasting cosmology , as would be delivered by cosmic string networks.

Garth
 
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  • #9
Is it not then possible that this 'extra' matter that we can't see exists in the form of black holes and not dark matter?
No, it’s not possible because scientists confirmed the existence and positional mapping of dark matter by a method called gravitational lensing. They send light rays in the space to locate dark matter. When light rays pass through a glass, they get bent. Similarly dark matter bends the light rays. Even though we can’t see dark matter, the bending and distortion of light rays tell us there is ‘some thing’ which is causing it. This ‘some thing’ is dark matter. A black hole can’t be dark matter for if we send light rays to a black hole, those light rays can never escape its gravitation. Like black holes, dark matter is not an eater of light. It only bends light.
 
  • #10
mdnazmulh BHs do bend light rays and would cause gravitational lensing.

See Stephen Brooks Website for simulated illustrations.

Garth
 
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  • #12
Some might think the standard [itex]\Lambda[/itex]CDM model looks a bit odd with its requirement of Inflation, DM and DE, all of which are undiscovered (so far) in laboratory experiments!

There may be a lot of problems with the coasting cosmology model, however you have to remember it has a lot more baryonic mass, and if the DM is dark baryonic material, say in the form of BHs, then a cloud of BHs would form galactic haloes and attract the visible matter as in the standard model. ( Non-interacting, etc.)

Garth
 
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  • #13
mdnazmulh said:
No, it’s not possible because scientists confirmed the existence and positional mapping of dark matter by a method called gravitational lensing. They send light rays in the space to locate dark matter. When light rays pass through a glass, they get bent. Similarly dark matter bends the light rays. Even though we can’t see dark matter, the bending and distortion of light rays tell us there is ‘some thing’ which is causing it. This ‘some thing’ is dark matter. A black hole can’t be dark matter for if we send light rays to a black hole, those light rays can never escape its gravitation. Like black holes, dark matter is not an eater of light. It only bends light.


Won't light be lensed by a black hole in much the same way? Light passing near the event horizon but not beyond it should lens in a very similar way, no?
 
  • #14
Universe black hole density...


The Universe black_hole density should be considered 'baryonic' because of theoretical Hawking radiation.

Universe black_hole density with respect to Milky Way galaxy:
[tex]\Omega_{bh} = \left( \frac{M_{bh}}{M_{MW}} \right) \Omega_b = 3.38 \cdot 10^{-7}[/tex]

[tex]\boxed{\Omega_{bh} = 3.38 \cdot 10^{-7}}[/tex]

[tex]M_{bh}[/tex] - Milky Way galactic nucleus mass
[tex]M_{MW}[/tex] - Milky Way galaxy mass
[tex]\Omega_b[/tex] - Universe baryon density

Reference:
http://en.wikipedia.org/wiki/Black_hole#Galactic_nuclei"
http://en.wikipedia.org/wiki/Milky_Way" [Broken]
http://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters"
 
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  • #15
mintparasol said:
Won't light be lensed by a black hole in much the same way? Light passing near the event horizon but not beyond it should lens in a very similar way, no?

That is correct. The only thing required for lensing is mass.
 
  • #16
Garth said:
Some might think the standard [itex]\Lambda[/itex]CDM model looks a bit odd with its requirement of Inflation, DM and DE, all of which are undiscovered (so far) in laboratory experiments!

Yes, but the oddness has been quantified, and after taking some sledgehammers to the theory, you end up with something that reproduces observations. I rather seriously doubt that if you take the coasting model as is, that you are going to be able to reproduce the galactic power spectrum or the Hubble measures. You may be able to get somewhere by taking a sledgehammer to the theory, but if you end up with more unknown entities it doesn't help.

There may be a lot of problems with the coasting cosmology model, however you have to remember it has a lot more baryonic mass, and if the DM is dark baryonic material, say in the form of BHs, then a cloud of BHs would form galactic haloes and attract the visible matter as in the standard model. ( Non-interacting, etc.)

Which is the the problem. If you have baryonic matter it interacts more strongly with visible matter so you end up with a power spectrum that is much less "fluffy" than with cold dark matter. Also a lot of the power spectrum calculations model things as sound waves through dark matter. If you have dark matter in black holes, things will look different...

Also there is the matter of converting the baryonic matter to black holes. If you do with the standard supernova formation, then you already have to have the galaxies form because you end up with black holes, and you should also be able to see massive amounts of star formation. If you do it through some non-standard black hole mechanism, it would help a lot of people mentioned what that mechanism might be.

One thing about these sort of objections is that they could be overcome. If you do a simple coasting universe model, and it happens to replicate one aspect of the standard model, that's a paper. The trouble is that if you do a coasting universe model, and then work through the numbers and it doesn't work out, that's not publishable.
 
  • #17
Yes twofish-quant, I agree there are a lot of problems with the freely coasting model although there are a few people (mainly in India) working on it who think they may be overcome such as: A Concordant “Freely Coasting” Cosmology.

It has a number of positive features too, such as: no need for Inflation, a ready explanation why the age of the universe happens to be exactly Hubble time and a solution to a possible age problem in the early universe.

The important thing (IMHO) for cosmology is to have alternative models against which the standard model may be compared and tested.

Garth
 
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  • #18
DaveC426913 said:
That is correct. The only thing required for lensing is mass.

So why dark matter then if we're saying that lensing leads us to postulate dark matter in the first place? Black holes are the most massive objects in the known universe. Why postulate dark matter if we already have the culprit nabbed?
 
  • #19
mintparasol said:
Black holes are the most massive objects in the known universe.

They aren't. If you take a 2 solar mass star and then turn it into a black hole. It's still 2 solar masses. If the black hole was formed by collapse of massive amounts of normal matter than you have a problem since it seems that dark matter isn't "normal matter" and you have to assume a massive amount of star formation in the early universe. If black holes aren't "normal matter" then you have an even bigger mystery of explaining how those black holes formed.

The other problem is that the black holes are in the wrong place. It appears that most galaxies have a massive supergiant black hole in the middle of them, but the galaxy rotation curves say that there is missing mass in the edges. Since we haven't seem supermassive black holes anywhere other than at the center of galaxies, we have a mystery.
 
  • #20
The mass in black holes formed from collapsed stars originally had to be baryonic (hydrogen/helium etc.) in nature.

The baryonic density limit of about 4% total (closure) density in standard BBN is not enough to create sufficient BH's in the standard model. The DM in that model (another 23% closure density) therefore has to consist of more exotic, non-baryonic massive particles, which are as yet undiscovered.

Given the undiscovered nature of 96% of the uiverse in that standard model, I like to keep an open mind on other possibilites, such as is provided by the lineraly expanding - coasting cosmology model.

Garth
 
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  • #21
Garth said:
The important thing (IMHO) for cosmology is to have alternative models against which the standard model may be compared and tested.

It's also important to have a standard model that you use as a benchmark for alternative models. In any case, today's alternative models turn into tomorrow's standard models.

This does point out one problem in that "things that don't work" don't get published. I'd be really interested to see what sort of power spectrum you come up with if you have a coasting model. It may be that no one has modified CMBFAST to deal with a coasting universe. It may be that someone has, they ran it, and what they got looked nothing at all like observations in which case you sunk three to six months and gotten nothing publishable. Things that don't work is the sort of stuff you find out over drinks at AAS meetings.
 
  • #22
Garth said:
I like to keep an open mind on other possibilites, such as is provided by the lineraly expanding - coasting cosmology model.

Same here. However you run into a problem in that there are dozens of alternative cosmologies, and going from anyone of those to get a nucleosynthesis prediction or a galactic power spectrum is very, very hard work.
 
  • #23
The mass in black holes formed from collapsed stars originally had to be baryonic (hydrogen/helium etc.) in nature.

If I read the EROS-2 result correctly, Black Holes with less than ~10 solar masses are excluded as DM candidates. How massive should these Population III Black Holes be?
 
  • #24
model parameters...


LCDM model parameters:
[tex]\Omega_b = 0.0456 \pm 0.0015[/tex] - baryon density
[tex]\Omega_c = 0.228 \pm 0.013[/tex] - dark matter density
[tex]\Omega_m = 0.2736[/tex] - total matter density (visible and Dark)
[tex]\Omega_{\Lambda} = 0.726 \pm 0.015[/tex] - dark energy density

SSC model parameters:
[tex]\Omega_m = 0.22[/tex] - total matter density (visible and Dark)
[tex]\Omega_{\Lambda} = 0.11[/tex] - false vacuum energy density

SSC coasting cosmology model parameter:
[tex]\Omega_b = 0.2[/tex] - baryon density

[tex]\Omega_b = \frac{8 \pi G \rho_o}{3 H_o^2}[/tex]

Wikipedia said:
In particular, as an example of a freely coasting model, in which the Universe expands strictly linearly with time, SCC clears basic constraints on nucleosynthesis, so there is no need to invoke unknown exotic non-baryonic DM. According to this theory the DM is originally baryonic in nature, this poses the major question for the theory: "In what form is this dark baryonic matter today?"

Reference:
http://en.wikipedia.org/wiki/Self-creation_cosmology#The_normal_Einstein_conformal_frame_cosmological_solution" [Broken]
http://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters"
http://arxiv.org/pdf/astro-ph/0306448v1"
 
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  • #25
Ich said:
If I read the EROS-2 result correctly, Black Holes with less than ~10 solar masses are excluded as DM candidates. How massive should these Population III Black Holes be?

IMBHs of mass ~ (102 - 103) MSolar

Garth
 
  • #26
IMBHs of mass ~ (10² - 10³) MSolar
As a SN remnant??
 
  • #27
Ich said:
As a SN remnant??

I would postulate two sources of BHs as DM: IMBHs as the remnants of Pop III stars (necessary in any model to explain ionisation and metallicity in the early universe) and much smaller primordial BHs as a remnant of primordial inhomogeneities in the BB itself.

Pop III star S/N could also be the source of deuterium in the linearly expanding model through spallation on shock fronts as that isotope would be lacking in that model's BBN. These S/N might also be the source of long duration GRB's.

This paper in today's physics arXiv may be pertinent: Baryons and Their Halos.
Abstract. Galaxies are composed of baryonic stars and gas embedded in dark matter halos. Here I briefly review two aspects of the connection between baryons and their halos. (1) The observed baryon content of galaxies falls short of the cosmic baryon fraction by an amount that varies
systematically with mass.Where these missing baryons now reside is unclear. (2) The characteristic acceleration in disk galaxies correlates strongly with their baryonic mass surface density. This implies a close coupling between the gravitational dynamics, which is presumably dominated by dark matter, and the purely baryonic components of galaxies.
From Figure 1 in that paper "The detected baryon fraction increases monotonically with mass while the stellar fraction peaks between M500 = 1012 and 1013 M."

From its conclusion:
We remain very far from being able to reproduce the observed coupling between dark and luminous matter in galaxy formation simulations. Indeed, this strikes me as a serious fine-tuning problem for any LCDM model. In this context, I find it disturbing that the only theory to have correctly predicted the observed behavior a priori is MOND

Not wanting to particularly advocate MOND, I would offer as an alternative explanation (to MOND) for this observed effect the postulate that the coupling between DM halos and baryon distribution is simply that they are both made the same 'stuff'.

This would require that stuff to be the (enhanced density) baryonic products of a linearly expanding BBN.

This explanantion would require that, as the original baryonic halo converted into a cloud of Pop III stars and subsequent IMBH's, the larger mass haloes/galaxies would not be so efficient at this process and leave a greater fraction of baryons to form visible matter - smaller Pop II and eventually Pop I stars.

Why the process would adopt this IMF I have no idea (yet)! Garth
 
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  • #28
Garth said:
Pop III star S/N could also be the source of deuterium in the linearly expanding model through spallation on shock fronts as that isotope would be lacking in that model's BBN.

Pretty skeptical that this is going to work. Deuterium is very hard to produce, and supernova shock fronts are just the wrong environment to even try to produce deuterium. Any shock front is just going to burn deuterium.

These S/N might also be the source of long duration GRB's.

Maybe, but again pretty skeptical that this is going to work.

This explanantion would require that, as the original baryonic halo converted into a cloud of Pop III stars and subsequent IMBH's, the larger mass haloes/galaxies would not be so efficient at this process and leave a greater fraction of baryons to form visible matter - smaller Pop II and eventually Pop I stars.

Why the process would adopt this IMF I have no idea (yet)!

Oh... That part is easy. You just assume that without heavy elements that the IMF is very, very different. Since we know very little about the IMF and how it changes with metallicity, it's not hard to wave your magic wand here. (But then it may be easy for me because I know something about supernova and nothing at all about ISM physics.)
 
  • #29
twofish-quant said:
Garth said:
Pop III star S/N could also be the source of deuterium in the linearly expanding model through spallation on shock fronts as that isotope would be lacking in that model's BBN.
Pretty skeptical that this is going to work. Deuterium is very hard to produce, and supernova shock fronts are just the wrong environment to even try to produce deuterium. Any shock front is just going to burn deuterium.
Agreed that there are problems! However I was 'hand-waving' towards this early paper by Epstein, Lattimer and Schramm : The Origin of Deuterium, Page 89, under the section 'Shock Waves' (Sorry I cannot copy and paste this). With many caveats they quote Colgate, Hoyle and Fowler to say that deuterium can be produced by a shock wave propagating through a low density medium. The problem they say would be the S/N required to produce the necessary shock wave would have to be "far greater than any observational inferences". I wonder whether Pop III S/N's might not do the trick...

twofish-quant said:
Garth said:
These S/N might also be the source of long duration GRB's.
Maybe, but again pretty skeptical that this is going to work.
Well that goes for the standard model too, Pop III stars are expected to end their lives in the early universe and long duration GRBs are observed - perhaps the two are linked?
twofish-quant said:
Garth said:
This explanantion would require that, as the original baryonic halo converted into a cloud of Pop III stars and subsequent IMBH's, the larger mass haloes/galaxies would not be so efficient at this process and leave a greater fraction of baryons to form visible matter - smaller Pop II and eventually Pop I stars.

Why the process would adopt this IMF I have no idea (yet)!
Oh... That part is easy. You just assume that without heavy elements that the IMF is very, very different. Since we know very little about the IMF and how it changes with metallicity, it's not hard to wave your magic wand here. (But then it may be easy for me because I know something about supernova and nothing at all about ISM physics.)
How would you hand-wave to explain the observed baryon-DM halo coupling?

Garth
 
  • #30
Garth said:
The problem they say would be the S/N required to produce the necessary shock wave would have to be "far greater than any observational inferences". I wonder whether Pop III S/N's might not do the trick...

I'm not seeing how that would work. The stronger the shock, the higher the post shock density and the higher the density, the less likely that you will end up with deuterium. This is all "gut feeling" so it could be wrong, but if it turns out that population III stars can produce lots of deuterium, that in itself would radically change the picture of how the universe evolved.

Well that goes for the standard model too, Pop III stars are expected to end their lives in the early universe and long duration GRBs are observed - perhaps the two are linked?

Or if you want something really wild and crazy. Let's go with "dark matter powered stars"

http://adsabs.harvard.edu/abs/2009ApJ...705.1031S

How would you hand-wave to explain the observed baryon-DM halo coupling?

You can imagine all sorts of mechanisms by which the existence of dark matter changes stellar evolution. More dark matter -> some sort of catalytic process that changes/replaces nuclear fusion -> more or fewer population III stars depending on what you need to make your model work...

http://adsabs.harvard.edu/abs/2008ApJ...688L...1Y

For example, increased dark matter density causes the first generation of stars to generate more heavy elements which then damps down further star formation.
 
  • #31
Thank you, I await the discovery of the WIMP DM candidate particle with anticipation!

As far as the last point is concerned, shouldn't it be the other way round? Do not the heavier haloes have increased star formation as a proportion of total mass? (According to the Baryons and Their Halos paper.)

Garth
 
  • #32
OK, I won't even pretend that I understand any of the math in this! My understanding of this is gleaned from high school physics (15 years ago) and a healthy interest in pop. science. Dark matter, or the notion of it, seems a bit like the emperor's new clothes to me. We can never say for sure how the universe we inhabit came to be. Sure, we can extrapolate by inputting observational data to extremely detailed and rigorous mathematical models what may have occurred in the very distant past but direct knowledge of it is clearly impossible. It seems to me that the unpredictable gravitational curves around massive galaxies could be caused by anything. The lensing and other observed effects which led to the postulation of dark matter may even be a property of spacetime itself, some as yet undetected property of a field or 'brane'.
Matter that is completely electromagnetically null? I'm not sold on this one...
 
  • #33
Matter that is completely electromagnetically null? I'm not sold on this one...
Well, then brace yourself for learning about http://en.wikipedia.org/wiki/Neutrino" [Broken].
emperor's new clothes
No, please. If you follow the discussions here, you'll find out why people believe that there is something more.
If it looks like cloth and feels like cloth, you'll go ahead with the preliminary assumption that the emperor is not naked.
If it turns out that he is naked: good bodypainting, and nothing one could have foreseen from pop sci.
 
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  • #34
mintparasol said:
Dark matter, or the notion of it, seems a bit like the emperor's new clothes to me.

Dark matter and dark energy reminds me of 19th century "ether", and there is a lot of stuff out there that is just screaming that we are all missing something fundamental. But I can't think of anything, lots of smart people are working on the problem, and they can't think of anything better. More data, more stumbling around in the dark, and eventually we'll figure something out.

We can never say for sure how the universe we inhabit came to be. Sure, we can extrapolate by inputting observational data to extremely detailed and rigorous mathematical models what may have occurred in the very distant past but direct knowledge of it is clearly impossible.

On the other hand, direct knowledge of anything is really impossible. I have this memory of being on the subway this morning. It could have been an illusion, and I could be in this giant matrix. Trying to figure out what happened 13 billion years ago, isn't all that much different from trying to figure out what happened this morning.

Also the models aren't that detailed and they certainly aren't very rigorous.

It seems to me that the unpredictable gravitational curves around massive galaxies could be caused by anything.

They can't be caused by *anything*. For example they can't be caused by non-dark matter since if it was caused by non-dark matter, we would have seen it. You eliminate what it can't be, then you try to make some sense about what it can be.

The lensing and other observed effects which led to the postulation of dark matter may even be a property of spacetime itself, some as yet undetected property of a field or 'brane'.

Maybe. At that point you go with that idea can then put limits on the properties of space time that it can't be.

Matter that is completely electromagnetically null? I'm not sold on this one...

Neutrons are electromagnetically null.
 
  • #35
twofish-quant said:
Neutrons are electromagnetically null.

They are detectable though, right? What's bugging me about dark matter is that it has never been directly detected, only postulated by inference.
 
<h2>1. What is dark matter?</h2><p>Dark matter is a type of matter that does not emit or interact with electromagnetic radiation, making it invisible to telescopes and other instruments. It is believed to make up about 85% of the total matter in the universe and is thought to play a crucial role in the formation and evolution of galaxies.</p><h2>2. How do scientists know that dark matter exists?</h2><p>Scientists have observed the effects of dark matter through its gravitational pull on visible matter in the universe. This can be seen in the rotation of galaxies, the bending of light from distant objects, and the large-scale structure of the universe. However, the exact nature of dark matter is still unknown.</p><h2>3. Why is dark matter called the "missing" matter?</h2><p>Dark matter is often referred to as the "missing" matter because it cannot be directly observed or detected. Its existence is inferred through its gravitational effects, but it remains elusive to scientists.</p><h2>4. Is dark matter hiding in plain sight in our own galaxy?</h2><p>There is evidence that suggests that a significant amount of dark matter may be present in our own galaxy, the Milky Way. However, due to its elusive nature, it is difficult to pinpoint its exact location or distribution within our galaxy.</p><h2>5. How are scientists searching for dark matter?</h2><p>Scientists are using a variety of methods to search for dark matter, including direct and indirect detection experiments, as well as studying the effects of dark matter on visible matter in the universe. Some experiments involve using detectors deep underground to try and capture particles of dark matter, while others involve studying the cosmic microwave background radiation for clues about dark matter's properties.</p>

1. What is dark matter?

Dark matter is a type of matter that does not emit or interact with electromagnetic radiation, making it invisible to telescopes and other instruments. It is believed to make up about 85% of the total matter in the universe and is thought to play a crucial role in the formation and evolution of galaxies.

2. How do scientists know that dark matter exists?

Scientists have observed the effects of dark matter through its gravitational pull on visible matter in the universe. This can be seen in the rotation of galaxies, the bending of light from distant objects, and the large-scale structure of the universe. However, the exact nature of dark matter is still unknown.

3. Why is dark matter called the "missing" matter?

Dark matter is often referred to as the "missing" matter because it cannot be directly observed or detected. Its existence is inferred through its gravitational effects, but it remains elusive to scientists.

4. Is dark matter hiding in plain sight in our own galaxy?

There is evidence that suggests that a significant amount of dark matter may be present in our own galaxy, the Milky Way. However, due to its elusive nature, it is difficult to pinpoint its exact location or distribution within our galaxy.

5. How are scientists searching for dark matter?

Scientists are using a variety of methods to search for dark matter, including direct and indirect detection experiments, as well as studying the effects of dark matter on visible matter in the universe. Some experiments involve using detectors deep underground to try and capture particles of dark matter, while others involve studying the cosmic microwave background radiation for clues about dark matter's properties.

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