# Could light be dark matter?

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? If that much is true, I wonder if this is significant enough to account for dark matter?

• 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 Jul 2 '14 at 12:21
• Light can be detected and Dark matter cannot be detected. – Yashbhatt Jul 2 '14 at 14:28
• 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 Jul 3 '14 at 2:12
• 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 Aug 5 '14 at 19:00
• 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. – Rob Jeffries Jun 22 '17 at 18:52

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.

• It appears light may be a gravity source as per the question / answers I linked to at the end of my question. – Jonathan Jul 2 '14 at 23:43
• 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 Jul 3 '14 at 3:37
• 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 Jul 3 '14 at 15:17
• Edited. Glad you found it explanatory enough. – harogaston Jul 3 '14 at 20:40
• 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. – Rob Jeffries Jun 22 '17 at 18:54

Most of the light energy in the universe is still in the cosmic microwave background (CMB). Spring 2011 UC Berkeley Physics 250 class materials calculate from the fact that $T=2.73$ for the CMB:

It follows that photons contribute only $0.0000485$ of the closure density.

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.

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.

• "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 Jul 3 '14 at 20:34
• @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 Jul 3 '14 at 22:24
• @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 Jul 4 '14 at 12:12

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).

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.

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...

• 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 Oct 21 at 22:01
• Best way to proceed would be for you to edit this and add in some additional information. – uhoh Oct 22 at 3:28

## protected by Mike GOct 21 at 21:01

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