<p>When you say "particle" candidates, I assume you're excluding <a href="http://en.wikipedia.org/wiki/MACHO" rel="noreferrer">MACHOs</a> and <a href="http://en.wikipedia.org/wiki/Robust_Associations_of_Massive_Baryonic_Objects" rel="noreferrer">RAMBOs</a>. MACHOs are "dark" objects at the stellar scale like black holes, neutron stars, brown dwarfs, etc. RAMBOs are clusters of similar dark objects. MACHOs and RAMBOs are made of primarily baryonic matter (everyday stuff like protons and neutrons &mdash; electrons are not baryons but they can be a part of this too). </p>

<p>This leaves us with the <a href="http://en.wikipedia.org/wiki/Weakly_interacting_massive_particle" rel="noreferrer">WIMPs</a> and possible other particle candidates. I don't know the details of any experiments looking for these offhand (I'll see if I can find it out and update the answer in a few days), but the major particle candidates are:</p>

<ul>
<li><a href="http://en.wikipedia.org/wiki/Neutrino" rel="noreferrer">Neutrinos</a>: It's highly unlikely that these make up any significant chunk of the DM in our universe, but they deserve a mention.</li>
<li><a href="http://en.wikipedia.org/wiki/Sterile_neutrino" rel="noreferrer">Sterile neutrinos</a> (I'm using the term in the specific sense here, not the general sense): This type of neutrino is right handed and only interacts gravitationally (note that there can be right handed active neutrinos, too). There is some preliminary evidence suggesting that these might exist.</li>
<li><a href="http://en.wikipedia.org/wiki/Axion" rel="noreferrer">Axions</a>: These are hypothetical, but if they exist (and make up the majority of DM), we kill two birds with one stone by both explaining DM and resolving the lack f symmetry breaking in the strong interaction.</li>
<li><a href="http://en.wikipedia.org/wiki/Gravitinos" rel="noreferrer">Gravitinos</a> and the <a href="http://en.wikipedia.org/wiki/Lightest_Supersymmetric_Particle" rel="noreferrer">Lightest Supersymmetric Partner</a>: Both of these require supersymmetry to hold, but they are major DM candidates.</li>
</ul>

CC BY-SA 3.0
2013-10-31T14:26:54.970Z
I thought dark matter does not interact except via gravity, didn't know it uses weak nuclear force as well. But that doesn't work at a distance. To see dark matter should be all about a choice of instruments. Ordinary telescopes, both earth-based and space-based, are sensitive to various parts of the electromagnetic spectrum, detecting photons emitted by cosmic objects. To see something that interacts only via gravity, a gravity telescope is needed that can see gravity waves emitted by dark matter/energy. http://lisa.nasa.gov/ is such an example. Could dark matter be non-particle things?
jmarina
CC BY-SA 3.0
2014-02-01T16:06:06.297Z
Many people believe that dark matter may in fact interact via the weak force as well, which is why a very long list of experiments have been built (and will be built) to look for this sort of signature. @jmarina, to answer your question about a "gravity telescope", you've got the right idea, but you're forgetting that light from background objects can be distorted by foreground mass - this is the basis for what's known as gravitational lensing and is currently one of our best methods of mapping out dark matter in the universe.
astromax
CC BY-SA 3.0
2013-09-27T15:53:38.600Z
Dark matter almost certainly does not interact in any way, shape, or form electromagnetically. For a while people thought that MACHOs were potentially the cause (whether they're brown dwarves or stellar mass black holes), but this has been ruled out through microlensing studies of the Mikly Way. However, there is a significant change that dark matter does interact via the weak force (which is why various types of neutrinos have been proposed - right-handed neutrinos, sterile neutrinos, etc..).
astromax
CC BY-SA 3.0
2013-09-27T15:57:39.630Z
Also, can you be more specific when you say "Neutrinos". There are a number of possibilities here.
astromax
CC BY-SA 3.0
2013-09-27T18:49:56.553Z
@astromax Aren't MACHOs still thought of as _part_ of the DM in the universe? I know that they're a minority, but IIRC they still are considered DM. You're right about the neutrino bit, I planned on adding a bit but forgot. Doing so now.
Manishearth
CC BY-SA 3.0
2013-09-27T18:59:02.940Z
Though MACHOs are tough to detect (since they're very faint; e.g. - brown dwarves, neutron stars, etc..), they are certainly made of baryonic content. Some work has been done to estimate the amount of mass due to objects like these, and it still doesn't even come close to adding up. Black holes, though nobody know what happens inside of the event horizon, also do not add up to what is needed to describe what galaxy/clusters dynamics and mass reconstructions from lensing, x-ray, and S-Z measurements tell us is there. So far as we know, gravity is the only confirmed interaction.
astromax
CC BY-SA 3.0
2013-09-27T19:01:29.977Z
@astromax Yep, I know. I'm just saying that they're still a part (however small) of the DM. The majority of the DM (where the interesting stuff happens) is with the WIMPs and similar species. Or am I wrong in categorizing MACHOs as a type of DM?
Manishearth
CC BY-SA 3.0
2013-09-27T19:04:52.440Z
I see what you're saying, but funamentally I believe (if it does turn out to be true that DM does not interact electromagnetically, which all evidence so far points to) that you are mis-categorizing things we simply have a hard time seeing as being part of the DM of the universe. If the DM particle is found, then you would be incorrect in saying a neutron star or a brown dwarf is composed of DM - they're not. We know what they're made of.
astromax
CC BY-SA 3.0
2013-09-27T19:16:51.427Z
@astromax Ah, I see. Well, the definition I've always known is that DM just needs to be nonluminous (this matches with the Wikipedia definition, though I don't trust that as much). I wouldn't say that neutron stars are _made of_ dark matter -- I don't think that DM needs to be particulate. Instead, I'm saying that some percentage of the nonluminous gravitating matter in the universe (i.e., what I call Dark Matter) is comprised of BHs and neutrons stars and whatnot.
Manishearth
CC BY-SA 3.0
2013-09-27T20:36:15.860Z
Well - things that are composed of composite particles with charge (protons/electrons, or quarks), radiate photons. Accelerating/decelerating charged particles produce radiation, so no, there is still a fundamental difference between the two.
astromax
CC BY-SA 3.0
2013-09-27T14:58:05.690Z
2013-09-28T13:53:42.350Z
2013-09-28T13:53:42.350Z
Manishearth
<p>Dark matter appears to dominate the matter component of the universe as compared to luminous, or baryonic, matter. Though it does not interact electromagnetically (it doesn't absorb, scatter, or emit photons), there is an ever-increasing mountain of evidence for its existence through it's gravitational interactions with stars, galaxies, and clusters, as well as it's influence on objects behind it through what's known as gravitational lensing. </p>

<p>My question is, what are the most promising particle candidates of dark matter, and which experiments currently exist (or will exist) to attempt to answer this question?</p>

<cosmology><dark-matter><matter><particles>
Dark Matter Particle Candidates
astromax
# Dark Matter Particle Candidates

(Asked by: username_0 on 2013-09-27)

Dark matter appears to dominate the matter component of the universe as compared to luminous, or baryonic, matter. Though it does not interact electromagnetically (it doesn't absorb, scatter, or emit photons), there is an ever-increasing mountain of evidence for its existence through it's gravitational interactions with stars, galaxies, and clusters, as well as it's influence on objects behind it through what's known as gravitational lensing.

My question is, what are the most promising particle candidates of dark matter, and which experiments currently exist (or will exist) to attempt to answer this question?

**Tags:** cosmology, dark-matter, matter, particles

---

## 1 answers

### Answer by username_1 on 2013-09-27. Score: 9

When you say "particle" candidates, I assume you're excluding [MACHOs](http://en.wikipedia.org/wiki/MACHO) and [RAMBOs](http://en.wikipedia.org/wiki/Robust_Associations_of_Massive_Baryonic_Objects). MACHOs are "dark" objects at the stellar scale like black holes, neutron stars, brown dwarfs, etc. RAMBOs are clusters of similar dark objects. MACHOs and RAMBOs are made of primarily baryonic matter (everyday stuff like protons and neutrons — electrons are not baryons but they can be a part of this too).

This leaves us with the [WIMPs](http://en.wikipedia.org/wiki/Weakly_interacting_massive_particle) and possible other particle candidates. I don't know the details of any experiments looking for these offhand (I'll see if I can find it out and update the answer in a few days), but the major particle candidates are:

* [Neutrinos](http://en.wikipedia.org/wiki/Neutrino): It's highly unlikely that these make up any significant chunk of the DM in our universe, but they deserve a mention.
* [Sterile neutrinos](http://en.wikipedia.org/wiki/Sterile_neutrino) (I'm using the term in the specific sense here, not the general sense): This type of neutrino is right handed and only interacts gravitationally (note that there can be right handed active neutrinos, too). There is some preliminary evidence suggesting that these might exist.
* [Axions](http://en.wikipedia.org/wiki/Axion): These are hypothetical, but if they exist (and make up the majority of DM), we kill two birds with one stone by both explaining DM and resolving the lack f symmetry breaking in the strong interaction.
* [Gravitinos](http://en.wikipedia.org/wiki/Gravitinos) and the [Lightest Supersymmetric Partner](http://en.wikipedia.org/wiki/Lightest_Supersymmetric_Particle): Both of these require supersymmetry to hold, but they are major DM candidates.

> **Comments:**
> **username_0**: Dark matter almost certainly does not interact in any way, shape, or form electromagnetically. For a while people thought that MACHOs were potentially the cause (whether they're brown dwarves or stellar mass black holes), but this has been ruled out through microlensing studies of the Mikly Way. However, there is a significant change that dark matter does interact via the weak force (which is why various types of neutrinos have been proposed - right-handed neutrinos, sterile neutrinos, etc..).  
> **username_0**: Also, can you be more specific when you say "Neutrinos". There are a number of possibilities here.  
> **username_1**: @astromax Aren't MACHOs still thought of as _part_ of the DM in the universe? I know that they're a minority, but IIRC they still are considered DM. You're right about the neutrino bit, I planned on adding a bit but forgot. Doing so now.  
> **username_0**: Though MACHOs are tough to detect (since they're very faint; e.g. - brown dwarves, neutron stars, etc..), they are certainly made of baryonic content. Some work has been done to estimate the amount of mass due to objects like these, and it still doesn't even come close to adding up. Black holes, though nobody know what happens inside of the event horizon, also do not add up to what is needed to describe what galaxy/clusters dynamics and mass reconstructions from lensing, x-ray, and S-Z measurements tell us is there. So far as we know, gravity is the only confirmed interaction.  
> **username_1**: @astromax Yep, I know. I'm just saying that they're still a part (however small) of the DM. The majority of the DM (where the interesting stuff happens) is with the WIMPs and similar species. Or am I wrong in categorizing MACHOs as a type of DM?  
> **username_0**: I see what you're saying, but funamentally I believe (if it does turn out to be true that DM does not interact electromagnetically, which all evidence so far points to) that you are mis-categorizing things we simply have a hard time seeing as being part of the DM of the universe. If the DM particle is found, then you would be incorrect in saying a neutron star or a brown dwarf is composed of DM - they're not. We know what they're made of.  
> **username_1**: @astromax Ah, I see. Well, the definition I've always known is that DM just needs to be nonluminous (this matches with the Wikipedia definition, though I don't trust that as much). I wouldn't say that neutron stars are _made of_ dark matter -- I don't think that DM needs to be particulate. Instead, I'm saying that some percentage of the nonluminous gravitating matter in the universe (i.e., what I call Dark Matter) is comprised of BHs and neutrons stars and whatnot.  
> **username_0**: Well - things that are composed of composite particles with charge (protons/electrons, or quarks), radiate photons. Accelerating/decelerating charged particles produce radiation, so no, there is still a fundamental difference between the two.  
> **username_2**: I thought dark matter does not interact except via gravity, didn't know it uses weak nuclear force as well. But that doesn't work at a distance. To see dark matter should be all about a choice of instruments. Ordinary telescopes, both earth-based and space-based, are sensitive to various parts of the electromagnetic spectrum, detecting photons emitted by cosmic objects. To see something that interacts only via gravity, a gravity telescope is needed that can see gravity waves emitted by dark matter/energy. http://lisa.nasa.gov/ is such an example. Could dark matter be non-particle things?  
> **username_0**: Many people believe that dark matter may in fact interact via the weak force as well, which is why a very long list of experiments have been built (and will be built) to look for this sort of signature. @jmarina, to answer your question about a "gravity telescope", you've got the right idea, but you're forgetting that light from background objects can be distorted by foreground mass - this is the basis for what's known as gravitational lensing and is currently one of our best methods of mapping out dark matter in the universe.  

---

