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? 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.


1 Answer 1


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%. But 2% is not 0%, so it is possible, and people are thinking about it. Example.

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 for the existence of dark matter.

  • $\begingroup$ 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. $\endgroup$
    – pela
    Commented Feb 6 at 9:55
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    $\begingroup$ @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? $\endgroup$
    – ProfRob
    Commented Feb 6 at 10:02
  • $\begingroup$ 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. $\endgroup$
    – Zinn
    Commented Feb 6 at 10:38
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    $\begingroup$ @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). $\endgroup$
    – Allure
    Commented Feb 6 at 13:24
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    $\begingroup$ @Allure Thanks, perhaps, but I would still need to dig deep into the literature. I think users like Sten or Kyle Kanos would be better suited… $\endgroup$
    – pela
    Commented Feb 6 at 15:18

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