<p>Ethan Siegel has a good discussion of this question <a href="https://www.forbes.com/sites/startswithabang/2019/03/07/how-much-of-the-dark-matter-could-neutrinos-be" rel="nofollow noreferrer">here</a>.  Read that for more detail, but briefly, the main issue is that neutrinos move too fast - they are a form of what cosmologists call &quot;hot dark matter&quot;.  The high speeds mean that they wouldn't settle into dense areas (e.g. galaxies, and clusters of galaxies) as dark matter is observed to do.   Evidence from structures we see in the universe tell us that <a href="https://science.sciencemag.org/content/322/5907/1476.full" rel="nofollow noreferrer">most dark matter must be cold</a>.</p>
<p>There certainly can be some hot dark matter in the form of neutrinos, but if there were a lot of it, it would leave an imprint on the cosmic microwave background that is not observed, constraining this component to be about 1% of the mass in the universe.</p>

CC BY-SA 4.0
2020-07-17T08:56:05.157Z
I see, and there is no way to slow down neutrinos to make it Cold Dark Matter. So, the shortest answer: its are too fast.
fstawyug4i
CC BY-SA 4.0
2020-07-17T09:21:48.417Z
But, imagine such scenario: Big-Bang - first days - universe is still dense. Its gravitational effect on neutrinos - is slowing its down. Enormous number of neutrinos, at first dark ages - becomes slow, because of density of universe. And after a while, they are simply flying around first starts, with normal speed, same as Earth's, thereafter around first black-holes, and Voila! - they are cold Dark Matter, slowed during Big-Bang, because of it... Too small to interact with matter, to slow to hit photons from atoms... not interacting at all with visible matter.
fstawyug4i
CC BY-SA 4.0
2020-07-17T10:14:43.797Z
@fstawyug4i The universe is dense everywhere -- gravity is pulling things equally in all directions. The expansion of the universe will slow neutrinos down to some extent, but we can calculate that, and they still end up relativistic.
Steve Linton
CC BY-SA 4.0
2020-07-17T00:24:08.797Z
2020-07-17T00:24:08.797Z
Eric Jensen
<p>What we know about neutrinos - its are almost invisible the same as Dark Matter, right?</p>
<p>Why Dark Matter could not consist of neutrinos?</p>

CC BY-SA 4.0
2020-11-23T12:47:42.587Z
See https://astronomy.stackexchange.com/questions/39945/could-not-slow-neutrinos-orbit-galaxies-and-clusters-thus-comprising-a-large-co
ProfRob
<dark-matter><neutrinos>
Could neutrinos be at least a part of Dark Matter?
fstawyug4i
# Could neutrinos be at least a part of Dark Matter?

(Asked by: username_0 on 2020-07-16)

What we know about neutrinos - its are almost invisible the same as Dark Matter, right?

Why Dark Matter could not consist of neutrinos?

**Tags:** dark-matter, neutrinos

> **Comments:**
> **username_1**: See https://astronomy.stackexchange.com/questions/39945/could-not-slow-neutrinos-orbit-galaxies-and-clusters-thus-comprising-a-large-co  

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## 1 answers

### Answer by username_2 on 2020-07-17. Score: 2 | Accepted answer

Ethan Siegel has a good discussion of this question [here](https://www.forbes.com/sites/startswithabang/2019/03/07/how-much-of-the-dark-matter-could-neutrinos-be). Read that for more detail, but briefly, the main issue is that neutrinos move too fast - they are a form of what cosmologists call "hot dark matter". The high speeds mean that they wouldn't settle into dense areas (e.g. galaxies, and clusters of galaxies) as dark matter is observed to do. Evidence from structures we see in the universe tell us that [most dark matter must be cold](https://science.sciencemag.org/content/322/5907/1476.full).

There certainly can be some hot dark matter in the form of neutrinos, but if there were a lot of it, it would leave an imprint on the cosmic microwave background that is not observed, constraining this component to be about 1% of the mass in the universe.

> **Comments:**
> **username_0**: I see, and there is no way to slow down neutrinos to make it Cold Dark Matter. So, the shortest answer: its are too fast.  
> **username_0**: But, imagine such scenario: Big-Bang - first days - universe is still dense. Its gravitational effect on neutrinos - is slowing its down. Enormous number of neutrinos, at first dark ages - becomes slow, because of density of universe. And after a while, they are simply flying around first starts, with normal speed, same as Earth's, thereafter around first black-holes, and Voila! - they are cold Dark Matter, slowed during Big-Bang, because of it... Too small to interact with matter, to slow to hit photons from atoms... not interacting at all with visible matter.  
> **username_3**: @username_0 The universe is dense everywhere -- gravity is pulling things equally in all directions. The expansion of the universe will slow neutrinos down to some extent, but we can calculate that, and they still end up relativistic.  

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