<p><strong>Unclear, but the operational assumption is that the amount of dark matter does not change.</strong></p>
<p>Modern cosmology is based on the Friedmann equations, for which the energy density of matter, <span class="math-container">$\Omega_m$</span>, is a key parameter. <span class="math-container">$\Omega_m$</span> varies over time as the universe expands, but it could in principle also vary if dark matter decays into other stuff (like radiation, measured by <span class="math-container">$\Omega_r$</span>). It could also in principle be possible to increase <span class="math-container">$\Omega_m$</span> if some non-matter thing forms dark matter.</p>
<p>However, observations indicate that <span class="math-container">$\Omega_m$</span> does not change much. <a href="https://arxiv.org/pdf/2011.01632.pdf" rel="noreferrer">The fraction of dark matter that can decay is less than about 2%</a>.
But 2% is not 0%, so it is possible, and people are thinking about it. <a href="https://arxiv.org/abs/2210.01303" rel="noreferrer">Example</a>.</p>
<p>Finally, it must be the case that the structure of dark matter changes with distance, because like other objects with mass, gravity affects how it moves. Structure formation is <a href="https://en.wikipedia.org/wiki/Dark_matter#Structure_formation" rel="noreferrer">one of the lines of evidence</a> for the existence of dark matter.</p>

CC BY-SA 4.0
2024-02-06T09:55:15.120Z
While I agree enough with this answer to +1 it, I tend to think that this answer somewhat underrates the overwhelming evidence we have that, even _if_ dark decay be able to decay, it must do so at an extremely low rate; low enough that observations of galactic dynamics, gravitational lensing, large-scale structure, etc. so far are consistent with no decay at all.
pela
CC BY-SA 4.0
2024-02-06T10:02:50.223Z
@pela good if you could write an answer that included those constraints. Do we know how well $\Omega_M(t)$ behaves, or is it more that we don't see see the products of dark matter decay?
ProfRob
CC BY-SA 4.0
2024-02-06T10:38:57.093Z
Thanks @allure. A couple of follow ups if you don’t mind:

1. “The energy density of matter, $\Omega_m$, varies over time as the universe expands” - because space is thought to be expanding, normal matter per area is decreasing?

2. “it might be possible that dark matter is formed from baryonic-matter interactions.” - meaning that dark matter could be accumulating over time, as opposed to decaying?

3. I’m curious as to how the answer could be ‘unclear’ when we could look at the surveys of dark matter and get a relationship between dark matter amount vs. distance form earth.
Zinn
CC BY-SA 4.0
2024-02-06T11:48:14.797Z
@ProfRob I was thinking more of the fact that rotation curves and other kinematical models, lensing mappings, N-body simulations etc. work just well in many aspects _without_ accounting for decaying dark matter, than I was thinking of the lack of direct evidence. But I don't think I have a good enough overview of the constraints to write a good answer.
pela
CC BY-SA 4.0
2024-02-06T13:22:47.720Z
@pela *But I don't think I have a good enough overview of the constraints to write a good answer* Maybe, but you likely have a better idea than me, so you can still write a better answer than I can =)
Allure
CC BY-SA 4.0
2024-02-06T13:24:49.247Z
@Zinn 1) Yes. The density of dark matter falls as the universe expands, while the density of dark energy stays constant. 2) In theory, yes. 3) What's unclear is whether dark matter decays. We don't know. The operational assumption is that it doesn't, but it could. If it does decay, it cannot be very fast (see pela's first comment).
Allure
CC BY-SA 4.0
2024-02-06T13:43:32.937Z
Thanks @allure. Something that interests me now is the fact that the Friedman equations that underpin all of cosmology are based on an uncertain assumption. A topic for another question maybe :)
Zinn
CC BY-SA 4.0
2024-02-06T13:47:19.997Z
@Zinn Hmm, I'll see if I can edit the answer to make it clearer what is meant. Friedmann equations does assume some things, but dark matter stability is not one of them.
Allure
CC BY-SA 4.0
2024-02-06T14:07:55.297Z
From your example reference it seems that the lower limit to any dark matter decay lifetimes is around $10^{14}$ years, so basically none of it will have decayed in $10^{10}$ years.
ProfRob
CC BY-SA 4.0
2024-02-06T15:18:31.423Z
@Allure Thanks, perhaps, but I would still need to dig deep into the literature. I think users like [Sten](https://astronomy.stackexchange.com/users/47607/sten) or [Kyle Kanos](https://astronomy.stackexchange.com/users/2079/kyle-kanos) would be better suited…
pela
CC BY-SA 4.0
2024-02-07T02:21:58.863Z
@Zinn My understanding is that **all of physics** is based on "uncertain" assumptions. One of the key things that experimental physicists do try to put ever-tighter bounds on **how much** uncertainty there are in those assumptions (i.e. that if value X was off by more than a certain amount we should have seen it).
Ben
CC BY-SA 4.0
2024-02-07T05:14:10.807Z
@Ben Fair point, but you have to admit there are vastly different levels to this uncertainty. The scientific community seems to still be in the speculation phase with regards to explaining the ‘missing matter’. So the fact that modern cosmology is based on equations involving measurements of matter we have no idea about is startling to me.
Zinn
CC BY-SA 4.0
2024-02-06T05:49:57.210Z
2024-02-06T13:58:55.903Z
2024-02-06T13:58:55.903Z
Allure
<p>In the hope that it may provide information on the development/evolution (if any) of dark matter over time, are there any differences (eg. in structure or concentration) in the dark matter at large radial distances from earth compared with closer radial distances.</p>
<p>I learnt from this question <a href="https://astronomy.stackexchange.com/questions/25925/how-does-the-amount-of-dark-matter-in-measured-galaxies-vary">How does the amount of dark matter in measured galaxies vary?</a> that “the smaller a galaxy is, the larger its amount of dark matter is”. But this appears to be due to the erosion of gas in the galaxies rather than developmental changes in the dark matter which I’m looking to understand.</p>

<cosmology><dark-matter>
Is the amount of dark matter the same as you look back through time (further away from earth)?
Zinn
# Is the amount of dark matter the same as you look back through time (further away from earth)?

(Asked by: username_0 on 2024-02-06)

In the hope that it may provide information on the development/evolution (if any) of dark matter over time, are there any differences (eg. in structure or concentration) in the dark matter at large radial distances from earth compared with closer radial distances.

I learnt from this question [How does the amount of dark matter in measured galaxies vary?](https://astronomy.stackexchange.com/questions/25925/how-does-the-amount-of-dark-matter-in-measured-galaxies-vary) that “the smaller a galaxy is, the larger its amount of dark matter is”. But this appears to be due to the erosion of gas in the galaxies rather than developmental changes in the dark matter which I’m looking to understand.

**Tags:** cosmology, dark-matter


---

## 1 answers

### Answer by username_1 on 2024-02-06. Score: 15 | Accepted answer

**Unclear, but the operational assumption is that the amount of dark matter does not change.**

Modern cosmology is based on the Friedmann equations, for which the energy density of matter, $\Omega\_m$, is a key parameter. $\Omega\_m$ varies over time as the universe expands, but it could in principle also vary if dark matter decays into other stuff (like radiation, measured by $\Omega\_r$). It could also in principle be possible to increase $\Omega\_m$ if some non-matter thing forms dark matter.

However, observations indicate that $\Omega\_m$ does not change much. [The fraction of dark matter that can decay is less than about 2%](https://arxiv.org/pdf/2011.01632.pdf).
But 2% is not 0%, so it is possible, and people are thinking about it. [Example](https://arxiv.org/abs/2210.01303).

Finally, it must be the case that the structure of dark matter changes with distance, because like other objects with mass, gravity affects how it moves. Structure formation is [one of the lines of evidence](https://en.wikipedia.org/wiki/Dark_matter#Structure_formation) for the existence of dark matter.

> **Comments:**
> **username_2**: While I agree enough with this answer to +1 it, I tend to think that this answer somewhat underrates the overwhelming evidence we have that, even _if_ dark decay be able to decay, it must do so at an extremely low rate; low enough that observations of galactic dynamics, gravitational lensing, large-scale structure, etc. so far are consistent with no decay at all.  
> **username_3**: @username_2 good if you could write an answer that included those constraints. Do we know how well $\Omega_M(t)$ behaves, or is it more that we don't see see the products of dark matter decay?  
> **username_0**: Thanks @username_1. A couple of follow ups if you don’t mind:  1. “The energy density of matter, $\Omega_m$, varies over time as the universe expands” - because space is thought to be expanding, normal matter per area is decreasing?  2. “it might be possible that dark matter is formed from baryonic-matter interactions.” - meaning that dark matter could be accumulating over time, as opposed to decaying?  3. I’m curious as to how the answer could be ‘unclear’ when we could look at the surveys of dark matter and get a relationship between dark matter amount vs. distance form earth.  
> **username_2**: @username_3 I was thinking more of the fact that rotation curves and other kinematical models, lensing mappings, N-body simulations etc. work just well in many aspects _without_ accounting for decaying dark matter, than I was thinking of the lack of direct evidence. But I don't think I have a good enough overview of the constraints to write a good answer.  
> **username_1**: @username_2 *But I don't think I have a good enough overview of the constraints to write a good answer* Maybe, but you likely have a better idea than me, so you can still write a better answer than I can =)  
> **username_1**: @username_0 1) Yes. The density of dark matter falls as the universe expands, while the density of dark energy stays constant. 2) In theory, yes. 3) What's unclear is whether dark matter decays. We don't know. The operational assumption is that it doesn't, but it could. If it does decay, it cannot be very fast (see username_2's first comment).  
> **username_0**: Thanks @username_1. Something that interests me now is the fact that the Friedman equations that underpin all of cosmology are based on an uncertain assumption. A topic for another question maybe :)  
> **username_1**: @username_0 Hmm, I'll see if I can edit the answer to make it clearer what is meant. Friedmann equations does assume some things, but dark matter stability is not one of them.  
> **username_3**: From your example reference it seems that the lower limit to any dark matter decay lifetimes is around $10^{14}$ years, so basically none of it will have decayed in $10^{10}$ years.  
> **username_2**: @username_1 Thanks, perhaps, but I would still need to dig deep into the literature. I think users like [Sten](https://astronomy.stackexchange.com/users/47607/sten) or [Kyle Kanos](https://astronomy.stackexchange.com/users/2079/kyle-kanos) would be better suited…  
> **username_4**: @username_0 My understanding is that **all of physics** is based on "uncertain" assumptions. One of the key things that experimental physicists do try to put ever-tighter bounds on **how much** uncertainty there are in those assumptions (i.e. that if value X was off by more than a certain amount we should have seen it).  
> **username_0**: @username_4 Fair point, but you have to admit there are vastly different levels to this uncertainty. The scientific community seems to still be in the speculation phase with regards to explaining the ‘missing matter’. So the fact that modern cosmology is based on equations involving measurements of matter we have no idea about is startling to me.  

---

