<p>Most of the light energy in the universe is still in the microwave background (MWB). The Wikipedia page on the MWB states its 2.7 K temperature implies it is making up a fraction of roughly <span class="math-container">$6 \times 10^{−5}$</span> of the total density of the universe.</p>
<p>Closure density is the density needed to close the universe if there were no cosmological constant. The observed density of the universe has been determined to be about 0.3 x closure density.</p>

CC BY-SA 4.0
2023-12-25T23:30:15.437Z
Your original source is still on wayback:  https://web.archive.org/web/20210507031959/https://www.int.washington.edu/PHYS554/2011/chapter2_11.pdf
James K
CC BY-SA 4.0
2023-12-27T21:11:39.740Z
I made a big change because the webpage that I referenced previously was removed.
eshaya
CC BY-SA 3.0
2014-10-07T22:18:16.473Z
Oddly enough, this also answers one of the other questions I had: http://astronomy.stackexchange.com/questions/4813/what-is-the-ratio-of-cosmic-microwave-background-radiation-to-normal-radiation  I would accept this as an answer to that question :-)
Jonathan
CC BY-SA 4.0
2014-10-06T22:38:24.190Z
2023-12-25T21:12:45.473Z
2023-12-25T21:12:45.473Z
eshaya
<p>As Yashbhatt said, we can detect light: with our eyes (visible light only) and with special machines. We can also see the effects of some type of lights. Dark matter, however, cannot be detected for now.</p>

<p>Also, light is energy, dark matter is matter. Why does your skin tan? It's because of the ultraviolet light. Why are you hot each summer? It's because of the infrared light. Light has an effect, so that's energy: your skin won't tan if there's no energy provided to make your skin tan.</p>

<p>In conclusion, light is not dark matter.</p>

CC BY-SA 3.0
2014-07-03T20:34:04.683Z
"dark matter is matter": Man we have absolutely no clue of what Dark Matter IS. Don't let the nomenclature mislead you. I'm sorry but you are just plain wrong there buddy.
harogaston
CC BY-SA 3.0
2014-07-03T22:24:32.310Z
@horagaston: 'matter' is a rather vague term with no completely consistent interdisciplinary meaning. As far as cosmology is concerned, dark matter is matter. In more abstract general relativity, one might even light to *also* be 'matter', but making a distinction between 'matter' and 'radiation' is more useful in cosmology. Instead, what I would take a slightly issue on is the claim that "light is energy"--it's a meaningless statement, because energy is a property of stuff, both light and matter.
Stan Liou
CC BY-SA 3.0
2014-07-04T12:12:03.727Z
@StanLiou well, for me saying "dark matter is matter" is wrong no *matter* which meaning you assign to it. Because we DON'T KNOW what dark matter IS. As soon as you define it as *something* you are being deceiving. But is it all right, I won't give it any more relevance than this.
harogaston
CC BY-SA 3.0
2014-07-02T21:32:45.260Z
2014-07-02T22:12:25.793Z
2014-07-02T22:12:25.793Z
Archa
<p>Light may account for a small portion of dark matter, but it is unlikely to account for most/all of it.</p>

<p>From a Wikipedia article on dark matter: <a href="http://en.wikipedia.org/wiki/Dark_matter" rel="nofollow">http://en.wikipedia.org/wiki/Dark_matter</a></p>

<blockquote>
  <p>the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3] Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total content of the universe.[4][5]</p>
</blockquote>

<p>Now that I think more about it, this would mean most of the mass in the universe would be light instead of matter if dark matter is light.  I doubt that most matter (e.g. in stars) turned into light, even throughout the life of the universe.  I believe hydrogen looses only a tiny portion of its mass (as energy / light) when it is fuzed into Helium.  However, it is possible that the cosmic background radiation, passing through almost all parts of the universe could account for more of the light.  I still doubt that would be enough to account for dark matter, but would like to see what effect this does have.  This tells me that light probably does not account for all dark matter, but that it could still account for a portion of it.  I welcome further input on this (e.g. how much of dark matter is light, if any).</p>

CC BY-SA 3.0
2014-07-03T11:53:29.083Z
2014-07-03T11:53:29.083Z
Jonathan
<p>Layman's explanation here, so take it for what it's worth.</p>

<p>Dark matter has a localized gravitational effect (localized to galaxies, yes, but localized nonetheless); its existence is hypothesized on the basis of this observed effect.</p>

<p>If dark matter were equally distributed throughout the universe, it could not have a localized effect, but instead its effect would be observed in the overall rate of the universe's expansion (or contraction); and at any rate it could not remain equally distributed, but would begin to clump together with the ordinary, visible matter.</p>

<p>So the dark matter has to clumped so that there's more of it in and around galaxies, and less of it in intergalactic space.</p>

<p>Light, however, could not clump in this way. While some of the light generated in a galaxy hits some of the matter in that galaxy and is absorbed, or hits a black hole in that galaxy and get stuck there until the end of time, for the most part the light emitted by a celestial body will make it to intergalactic space, and unless the universe is dense enough to eventually collapse, that light will never return to its home galaxy.</p>

<p>Furthermore&mdash;as others have pointed out&mdash;dark matter is postulated because the visible matter in galaxies is insufficient to explain its rotation. Since the visible matter has far and away more mass than the light emitted by that matter (by multiple orders of magnitude), but is insufficient to account for the rotation of galaxies, then surely the radically smaller mass of the emitted light must necessarily be even less sufficient, and thus could not be an explanation for what dark matter is.</p>

<p>So no, dark matter is not light.</p>

CC BY-SA 4.0
2019-01-15T06:47:53.020Z
2019-01-15T06:47:53.020Z
EvilSnack
<p>Light cannot be dark matter by definition. This is because dark matter is defined in a way as to exclude light as a candidate. <a href="https://en.wikipedia.org/wiki/Dark_matter#Technical_definition" rel="nofollow noreferrer">See the technical definition section of the Dark Matter article on Wikipedia</a>:</p>
<blockquote>
<p>In standard cosmologic calculations, &quot;matter&quot; means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., <span class="math-container">$\rho \propto a^{-3}$</span>. This is in contrast to &quot;radiation&quot;, which scales as the inverse fourth power of the scale factor <span class="math-container">$\rho \propto a^{−4}$</span> , and a cosmological constant, which does not change with respect to <span class="math-container">$a$</span> ...</p>
<p>In principle, &quot;dark matter&quot; means all components of the universe which are not visible but still obey <span class="math-container">$\rho \propto a^{-3}$</span>.</p>
</blockquote>
<p>Light, as radiation, obeys <span class="math-container">$\rho \propto a^{−4}$</span>. Matter on the other hand obeys <span class="math-container">$\rho \propto a^{−3}$</span>. Therefore light cannot be matter.</p>
<p>(Other answers illustrate how the energy content of light is already taken into account in standard cosmology. In the current universe, the amount of light is minute compared to matter &amp; dark energy.)</p>

CC BY-SA 4.0
2023-12-26T08:33:41.900Z
2023-12-26T08:33:41.900Z
Allure
<p>Light has certainly a gravitational mass. Light is also dispersed into space with the inverse of the square of distance. That means that its weight is stronger all along the galaxies disk where the light sources are. Just like dark matter is supposed to be. Local stronger light in the galaxies is to be added to all the light from intergalactic sources and CMB, which is more or less the same across the universe, and should have an effect on galaxies too. The point is how much the light in the whole EM spectrum in the interstellar space weights. It's clearly negligeable into solar systems near stars, compared to regular mass, but maybe not if you add all the light in the huge space between stars. Plus add the gravity of light in "empty" space between galaxies (galaxies rotation should be also affected with the gravity of light &amp; matter between galaxies).
I think astronomers must have taken this into account, but I can't find the calculated amount anywhere (I'm not an astronomer, maybe it's in many specialized astronomy books, I don't know). I suspect anyway that light weight is much, much less than required for being the dark matter, but I don't have the numbers. If anyone has them...</p>

CC BY-SA 4.0
2019-10-21T22:01:52.440Z
Hi Fernando, since you don't have the numbers, this is more of a comment than an answer. In fact, the numbers are readily available from the CMB temperature maps, and is easily shown to be negligible (some $10^{-5}$ of the total energy density). I suspect that's why your answer was downvoted.
pela
CC BY-SA 4.0
2019-10-22T03:28:33.067Z
Best way to proceed would be for you to edit this and add in some additional information.
uhoh
CC BY-SA 4.0
2019-10-21T20:59:18.820Z
2019-10-21T20:59:18.820Z
Fernando C.
<p>Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain.</p>

<p>The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is what binds galaxies together. </p>

<p>Furthermore, we have included everything we can observe (all ordinary matter including photons) when we do the calculations for the amount of gravity there should be. So light is already there. 'Dark matter' is that extra gravity which we cannot account for.</p>

CC BY-SA 3.0
2017-06-22T18:54:22.027Z
This is not correct. It is called "dark matter" both because it cannot be seen, but also because the majority of it needs to be in a form that does not interact electromagnetically. Therefore light cannot be dark matter.
ProfRob
CC BY-SA 3.0
2014-07-02T23:43:13.813Z
It appears light may be a gravity source as per the question / answers I linked to at the end of my question.
Jonathan
CC BY-SA 3.0
2014-07-03T03:37:44.340Z
Thank you for pointing that article out. As I read it, it is pretty clear that the gravity caused by light is ridiculously small, yet dark matter represents a gravity source even greater than that of ordinary matter. Furthermore, we include everything we can observe (all ordinary matter including photons) when we do the calculations for the amount of gravity. So it is already there. 'Dark matter' is that extra gravity for which we cannot account for.
harogaston
CC BY-SA 3.0
2014-07-03T15:17:10.160Z
I would consider that comment a good answer :-) If you modify your answer with this, if I don't get anything better, I will probably accept that as the answer.
Jonathan
CC BY-SA 3.0
2014-07-03T20:40:56.813Z
Edited. Glad you found it explanatory enough.
harogaston
CC BY-SA 3.0
2014-07-02T22:37:55.907Z
2014-07-03T20:40:16.407Z
2014-07-03T20:40:16.407Z
harogaston
<p>Is it possible that light itself is dark matter?  I am speaking of photons (e.g. visible light, infrared, ultraviolet, etc...).  I realize light is understood to be massless, but it is obvious it at least contains energy because we can see with it (e.g. it energizes the cells in our retinas).  I wonder if light has a very tiny "net mass" (e.g. 0 mass * relativistically infinite speed).  I would think that light at least has a little mass, in proportion to its energy.  For example, take E=mc^2, then m = E/c^2 would describe how much mass it has.  If this is true, light should have a very little gravity too.  Although the effect would seem minimal, light is practically everywhere. Gravity from light would be more concentrated inside galaxies, and even more concentrated in the center of galaxies where there are many stars (like dark matter is).  It would be interesting to run the calculations, assuming light does have gravity, and see if this matches the gravitational observations of dark matter in the universe.  It would be funny and ironic if dark matter really is light.</p>

<p>Edit: Note that it appears light does have gravity as per the discussions here: <a href="https://astronomy.stackexchange.com/questions/1333/how-does-light-affect-the-universe?rq=1">How does light affect the universe?</a>  If that much is true, I wonder if this is significant enough to account for dark matter?</p>

CC BY-SA 3.0
2016-02-08T20:42:38.263Z
This is pretty interesting. Some say, if it were light, we would see it, but would we? We don't see the photons that are coming off the sun, perpendicular to our line of sight. We don't see them as they pass us heading to Jupiter, but yet there they are, reflecting back to us. In a sense we see the effect of light, but not the light it self (like a laser). I always wondered if it was just that instead of admitting the mass of things is more than we thought, we say it has to be something else.
coblr
CC BY-SA 3.0
2016-02-08T21:45:56.383Z
@fractalspawn I agree with your analysis.  We don't see the light unless it enters our retinas (or whatever detection device we use).  This means there is a tremendous amount of light we do not see.  For example, we don't see a laser beam (light in transit), but we see the point where the beam strikes because light bounces off the object and then enters our eyes.
Jonathan
CC BY-SA 3.0
2017-06-22T18:52:07.680Z
Light is massless. Full stop. A better way to think about it is that energy contributes to the spatial curvature that is interpreted as the force of gravity. Either way, the current light content of the universe is a negligible source of energy density.
ProfRob
CC BY-SA 3.0
2014-07-02T12:21:28.970Z
This seems a little far-fetched and I would expect that if there was light, we would see it. Also, I believe that all cosmological models account for the effect of light/photons/radiation. However, this is an extremely interesting correlation or a line of thought that I think I am absolutely in love with. Being not very comfortable discussing details about cosmology myself, I would like to see any answers people have. So, a +1. Also, great thinking, keep it up. This is the kind of creativity that leads to great research.
Takku
CC BY-SA 3.0
2014-07-02T14:28:01.733Z
Light can be detected and Dark matter cannot be detected.
Yashbhatt
CC BY-SA 3.0
2014-07-03T02:12:11.060Z
An interesting question... if you add up all the energy of all the "in transit" photons in the universe (in some definition of "now") and equate it to mass via E=mc^2, is it a meaningful thing to do (does it behave as mass e.g. have gravity), is that mass of any significance, and if so, does it in any way account for the so-called "missing" or dark matter in our universe, even if only partially?
Anthony X
CC BY-SA 3.0
2014-07-03T05:53:32.383Z
In this way, you are assuming that light gravitates much more than ordinary matter.
Py-ser
CC BY-SA 3.0
2014-07-03T11:38:19.490Z
@AnthonyX It appears the answers and sources to the linked question show that light does gravitate according to the mass it would have by E=mc^2.  If this is true, the big question is, is this enough to account for dark matter?
Jonathan
CC BY-SA 3.0
2014-07-03T11:40:06.107Z
@Py-ser I would like to see how much of dark matter it accounts for, assuming the mass associated with light (m=E/c^2) gravitates just like the equivalent mass in normal matter.
Jonathan
CC BY-SA 3.0
2014-08-05T19:00:52.390Z
Interesting. One common idea has dark matter being a WIMP (Weakly Interacting Massive Particle). While photons don't seem to interact via the weak nuclear force, I wonder if electroweak unification could give the photon dark matter idea an interesting twist.
HDE 226868
<galaxy><gravity><light><dark-matter><galaxy-center>
Could light be dark matter?
Jonathan
# Could light be dark matter?

(Asked by: username_0 on 2014-07-02)

Is it possible that light itself is dark matter? I am speaking of photons (e.g. visible light, infrared, ultraviolet, etc...). I realize light is understood to be massless, but it is obvious it at least contains energy because we can see with it (e.g. it energizes the cells in our retinas). I wonder if light has a very tiny "net mass" (e.g. 0 mass \* relativistically infinite speed). I would think that light at least has a little mass, in proportion to its energy. For example, take E=mc^2, then m = E/c^2 would describe how much mass it has. If this is true, light should have a very little gravity too. Although the effect would seem minimal, light is practically everywhere. Gravity from light would be more concentrated inside galaxies, and even more concentrated in the center of galaxies where there are many stars (like dark matter is). It would be interesting to run the calculations, assuming light does have gravity, and see if this matches the gravitational observations of dark matter in the universe. It would be funny and ironic if dark matter really is light.

Edit: Note that it appears light does have gravity as per the discussions here: [How does light affect the universe?](https://astronomy.stackexchange.com/questions/1333/how-does-light-affect-the-universe?rq=1) If that much is true, I wonder if this is significant enough to account for dark matter?

**Tags:** galaxy, gravity, light, dark-matter, galaxy-center
> **Comments:**
> **username_3**: This seems a little far-fetched and I would expect that if there was light, we would see it. Also, I believe that all cosmological models account for the effect of light/photons/radiation. However, this is an extremely interesting correlation or a line of thought that I think I am absolutely in love with. Being not very comfortable discussing details about cosmology myself, I would like to see any answers people have. So, a +1. Also, great thinking, keep it up. This is the kind of creativity that leads to great research.  
> **username_4**: Light can be detected and Dark matter cannot be detected.  
> **username_5**: An interesting question... if you add up all the energy of all the "in transit" photons in the universe (in some definition of "now") and equate it to mass via E=mc^2, is it a meaningful thing to do (does it behave as mass e.g. have gravity), is that mass of any significance, and if so, does it in any way account for the so-called "missing" or dark matter in our universe, even if only partially?  
> **username_6**: In this way, you are assuming that light gravitates much more than ordinary matter.  
> **username_0**: @AnthonyX It appears the answers and sources to the linked question show that light does gravitate according to the mass it would have by E=mc^2.  If this is true, the big question is, is this enough to account for dark matter?  
> **username_0**: @Py-ser I would like to see how much of dark matter it accounts for, assuming the mass associated with light (m=E/c^2) gravitates just like the equivalent mass in normal matter.  
> **username_7**: Interesting. One common idea has dark matter being a WIMP (Weakly Interacting Massive Particle). While photons don't seem to interact via the weak nuclear force, I wonder if electroweak unification could give the photon dark matter idea an interesting twist.  
> **username_1**: This is pretty interesting. Some say, if it were light, we would see it, but would we? We don't see the photons that are coming off the sun, perpendicular to our line of sight. We don't see them as they pass us heading to Jupiter, but yet there they are, reflecting back to us. In a sense we see the effect of light, but not the light it self (like a laser). I always wondered if it was just that instead of admitting the mass of things is more than we thought, we say it has to be something else.  
> **username_0**: @fractalspawn I agree with your analysis.  We don't see the light unless it enters our retinas (or whatever detection device we use).  This means there is a tremendous amount of light we do not see.  For example, we don't see a laser beam (light in transit), but we see the point where the beam strikes because light bounces off the object and then enters our eyes.  
> **username_2**: Light is massless. Full stop. A better way to think about it is that energy contributes to the spatial curvature that is interpreted as the force of gravity. Either way, the current light content of the universe is a negligible source of energy density.  

---

## 7 answers

### Answer by username_11 on 2014-07-02. Score: 28 | Accepted answer

Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain.

The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is what binds galaxies together.

Furthermore, we have included everything we can observe (all ordinary matter including photons) when we do the calculations for the amount of gravity there should be. So light is already there. 'Dark matter' is that extra gravity which we cannot account for.

> **Comments:**
> **username_0**: It appears light may be a gravity source as per the question / answers I linked to at the end of my question.  
> **username_11**: Thank you for pointing that article out. As I read it, it is pretty clear that the gravity caused by light is ridiculously small, yet dark matter represents a gravity source even greater than that of ordinary matter. Furthermore, we include everything we can observe (all ordinary matter including photons) when we do the calculations for the amount of gravity. So it is already there. 'Dark matter' is that extra gravity for which we cannot account for.  
> **username_0**: I would consider that comment a good answer :-) If you modify your answer with this, if I don't get anything better, I will probably accept that as the answer.  
> **username_11**: Edited. Glad you found it explanatory enough.  
> **username_2**: This is not correct. It is called "dark matter" both because it cannot be seen, but also because the majority of it needs to be in a form that does not interact electromagnetically. Therefore light cannot be dark matter.  

---

### Answer by username_8 on 2014-10-06. Score: 10

Most of the light energy in the universe is still in the microwave background (MWB). The Wikipedia page on the MWB states its 2.7 K temperature implies it is making up a fraction of roughly $6 \times 10^{−5}$ of the total density of the universe.

Closure density is the density needed to close the universe if there were no cosmological constant. The observed density of the universe has been determined to be about 0.3 x closure density.

> **Comments:**
> **username_0**: Oddly enough, this also answers one of the other questions I had: http://astronomy.stackexchange.com/questions/4813/what-is-the-ratio-of-cosmic-microwave-background-radiation-to-normal-radiation  I would accept this as an answer to that question :-)  
> **username_9**: Your original source is still on wayback:  https://web.archive.org/web/20210507031959/https://www.int.washington.edu/PHYS554/2011/chapter2_11.pdf  
> **username_8**: I made a big change because the webpage that I referenced previously was removed.  

---

### Answer by username_10 on 2014-07-02. Score: 5

As Yashbhatt said, we can detect light: with our eyes (visible light only) and with special machines. We can also see the effects of some type of lights. Dark matter, however, cannot be detected for now.

Also, light is energy, dark matter is matter. Why does your skin tan? It's because of the ultraviolet light. Why are you hot each summer? It's because of the infrared light. Light has an effect, so that's energy: your skin won't tan if there's no energy provided to make your skin tan.

In conclusion, light is not dark matter.

> **Comments:**
> **username_11**: "dark matter is matter": Man we have absolutely no clue of what Dark Matter IS. Don't let the nomenclature mislead you. I'm sorry but you are just plain wrong there buddy.  
> **username_12**: @horagaston: 'matter' is a rather vague term with no completely consistent interdisciplinary meaning. As far as cosmology is concerned, dark matter is matter. In more abstract general relativity, one might even light to *also* be 'matter', but making a distinction between 'matter' and 'radiation' is more useful in cosmology. Instead, what I would take a slightly issue on is the claim that "light is energy"--it's a meaningless statement, because energy is a property of stuff, both light and matter.  
> **username_11**: @StanLiou well, for me saying "dark matter is matter" is wrong no *matter* which meaning you assign to it. Because we DON'T KNOW what dark matter IS. As soon as you define it as *something* you are being deceiving. But is it all right, I won't give it any more relevance than this.  

---

### Answer by username_0 on 2014-07-03. Score: 3

Light may account for a small portion of dark matter, but it is unlikely to account for most/all of it.

From a Wikipedia article on dark matter: <http://en.wikipedia.org/wiki/Dark_matter>

> the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3] Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total content of the universe.[4][5]

Now that I think more about it, this would mean most of the mass in the universe would be light instead of matter if dark matter is light. I doubt that most matter (e.g. in stars) turned into light, even throughout the life of the universe. I believe hydrogen looses only a tiny portion of its mass (as energy / light) when it is fuzed into Helium. However, it is possible that the cosmic background radiation, passing through almost all parts of the universe could account for more of the light. I still doubt that would be enough to account for dark matter, but would like to see what effect this does have. This tells me that light probably does not account for all dark matter, but that it could still account for a portion of it. I welcome further input on this (e.g. how much of dark matter is light, if any).


---

### Answer by username_13 on 2019-01-15. Score: 2

Layman's explanation here, so take it for what it's worth.

Dark matter has a localized gravitational effect (localized to galaxies, yes, but localized nonetheless); its existence is hypothesized on the basis of this observed effect.

If dark matter were equally distributed throughout the universe, it could not have a localized effect, but instead its effect would be observed in the overall rate of the universe's expansion (or contraction); and at any rate it could not remain equally distributed, but would begin to clump together with the ordinary, visible matter.

So the dark matter has to clumped so that there's more of it in and around galaxies, and less of it in intergalactic space.

Light, however, could not clump in this way. While some of the light generated in a galaxy hits some of the matter in that galaxy and is absorbed, or hits a black hole in that galaxy and get stuck there until the end of time, for the most part the light emitted by a celestial body will make it to intergalactic space, and unless the universe is dense enough to eventually collapse, that light will never return to its home galaxy.

Furthermore—as others have pointed out—dark matter is postulated because the visible matter in galaxies is insufficient to explain its rotation. Since the visible matter has far and away more mass than the light emitted by that matter (by multiple orders of magnitude), but is insufficient to account for the rotation of galaxies, then surely the radically smaller mass of the emitted light must necessarily be even less sufficient, and thus could not be an explanation for what dark matter is.

So no, dark matter is not light.


---

### Answer by username_14 on 2023-12-26. Score: 0

Light cannot be dark matter by definition. This is because dark matter is defined in a way as to exclude light as a candidate. [See the technical definition section of the Dark Matter article on Wikipedia](https://en.wikipedia.org/wiki/Dark_matter#Technical_definition):

> In standard cosmologic calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., $\rho \propto a^{-3}$. This is in contrast to "radiation", which scales as the inverse fourth power of the scale factor $\rho \propto a^{−4}$ , and a cosmological constant, which does not change with respect to $a$ ...
>
> In principle, "dark matter" means all components of the universe which are not visible but still obey $\rho \propto a^{-3}$.

Light, as radiation, obeys $\rho \propto a^{−4}$. Matter on the other hand obeys $\rho \propto a^{−3}$. Therefore light cannot be matter.

(Other answers illustrate how the energy content of light is already taken into account in standard cosmology. In the current universe, the amount of light is minute compared to matter & dark energy.)


---

### Answer by username_15 on 2019-10-21. Score: -2

Light has certainly a gravitational mass. Light is also dispersed into space with the inverse of the square of distance. That means that its weight is stronger all along the galaxies disk where the light sources are. Just like dark matter is supposed to be. Local stronger light in the galaxies is to be added to all the light from intergalactic sources and CMB, which is more or less the same across the universe, and should have an effect on galaxies too. The point is how much the light in the whole EM spectrum in the interstellar space weights. It's clearly negligeable into solar systems near stars, compared to regular mass, but maybe not if you add all the light in the huge space between stars. Plus add the gravity of light in "empty" space between galaxies (galaxies rotation should be also affected with the gravity of light & matter between galaxies).
I think astronomers must have taken this into account, but I can't find the calculated amount anywhere (I'm not an astronomer, maybe it's in many specialized astronomy books, I don't know). I suspect anyway that light weight is much, much less than required for being the dark matter, but I don't have the numbers. If anyone has them...

> **Comments:**
> **username_16**: Hi Fernando, since you don't have the numbers, this is more of a comment than an answer. In fact, the numbers are readily available from the CMB temperature maps, and is easily shown to be negligible (some $10^{-5}$ of the total energy density). I suspect that's why your answer was downvoted.  
> **username_17**: Best way to proceed would be for you to edit this and add in some additional information.  

---

