I've read that dark matter's existence is theorized because galaxies spin/clump in a way that would require much more mass than there appears to be. Also that there is gravitational lensing in places where there does not appear to be as much mass as would be required for the amount of lensing observed. So it is assumed there is some mass that we can't see, which is called dark matter.

Is there any evidence about how long dark matter has been around? Has the amount of dark matter been constant since the big bang or is it increasing? If it is increasing, is regular matter decreasing at an equal rate?


How long has dark matter been around?

Short Answer

Almost forever.

Long Answer

Assuming a dark matter particle paradigm, according to a pre-print by Yang (2015) subsequently published in Physical Review D, the lower bound on the mean lifetime of dark matter particles is $3.57\times 10^{24}$ seconds. This is roughly $10^{17}$ years. By comparison the age of the universe is roughly $1.38 \times 10^{10}$ years.

This means that dark matter (if it exists) is at least as stable as anything other than a proton, which has an experimentally determined mean lifetime of at least $10^{34}$ years, or an electron, which is theoretically stable (just as the proton is in the Standard Model) and has an experimentally determined mean lifetime of at least $6.6×10^{28}$ years.

This means that all dark matter candidates that are not perfectly stable or at least metastable are ruled out. Decaying dark matter and dark matter with any significant annihilation cross section are inconsistent with observation, unless there is a mechanism that generates new dark matter in equilibrium with the amount annihilated.

Has the amount of dark matter been constant since the big bang or is it increasing? If it is increasing, is regular matter decreasing at an equal rate?

The LambdaCDM "Standard Model of Cosmology" assumes a constant amount of dark matter in the universe after the earliest moments of the universe (with the density of the dark matter in the universe decreasing in proportion to the spatial volume of the universe), just as the model does in the case of ordinary baryonic matter.

For purposes of this question, exactly how many moments after the Big Bang it takes for dark matter to emerge is pretty much irrelevant, as this number is much smaller (by a factor of many billions) than margin of error in our estimates of the age of the universe.


Not all lines of evidence are consistent with this analysis, however. An article in the journal Nature, Bowman (March 2018), analyzing the "21 centimeter line" in the radio spectrum finds that:

[E]ither the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude.

In other words, this evidence contradicts the LambdaCDM model, which assumes that dark matter is "almost collisionless" and hence could not cause massive cooling through interactions between dark matter and baryons. This evidence is consistent with an early universe (post-radiation era, hundreds of millions of years after the Big Bang) that has no dark matter, but that possibility throws a wrench into other aspects of the LambdaCDM model.

This contradiction, just recognized a few months ago, has not yet been adequately resolved.

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    $\begingroup$ @StephenG I would if I could, but that is beyond my skill set. I'd welcome edits reformatting the numbers from someone who is able to do so. In my day through an undergraduate math major, all math was done with pen and paper and I never learned any other method of formatting, although they apparently now teach LaTeX typesetting in my son's high school math classes. Physics Forums has an easier interface for that kind of thing (at least superscripts and subscripts and Greek letters), but I don't have the patience to learn Mathjax at this point in my life, given how infrequently I would use it. $\endgroup$ – ohwilleke Jul 31 '18 at 5:23
  • $\begingroup$ Be aware that Mathjax is the site standard for mathematical expressions and it's common for non-Mathjax postings using maths to get down votes. And while learning all of mathjax might be tedious, you don't need most of it to get the basics. It's almost identical to Latex, BTW. $\endgroup$ – StephenG Jul 31 '18 at 5:34
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    $\begingroup$ My brief research suggests that dark matter annihilation is very much an ongoing research topic and rather poorly constrained by observation. e.g. arxiv.org/abs/1804.01055 $\endgroup$ – Rob Jeffries Jul 31 '18 at 10:58
  • $\begingroup$ @RobJeffries I don't disagree that dark matter annihilation is an ongoing research tropic, but this is rather less because it is poorly constrained by observation and rather more because the field is full of zombie theories that persist because investigators tend to consider their own discipline when determining if a candidate theory has been ruled out, even when other lines of evidence of a different type strongly disfavor a theory. For example, astronomy data has strongly disfavored relic DM candidates of 1 GeV or more based on galactic dynamics, but these theories continue to be proposed. $\endgroup$ – ohwilleke Jul 31 '18 at 16:42
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    $\begingroup$ @jpmc26 Suggestion taken. I was paraphrasing some language from the body text of the reference, but in the process it got pretty ugly, I admit. $\endgroup$ – ohwilleke Jul 31 '18 at 22:18

It depends upon which dark matter theory you follow: some dark matter theories predict that dark matter particles have rest masses in TeVs (super-WIMP), while others predict masses in keVs (certain Warm Dark Matter-WDM-candidates).

The expected rest mass of WIMPs is in GeV-TeVs, so we'd need an extremely high temperature and density for their formation, which could only have been achieved in the early universe. It's unlikely that appropriate conditions are abundant now, even during supernovae (see this physics SE post: When and where was WIMP dark matter formed?). But clearly it'd be easier to achieve the most basic condition: energy availability, needed for the formation of lighter particles (there may be other factors to consider as well). For example, if sterile neutrino theories (among other WDMs) are correct, there could conceivably be a lot of dark matter formation going on (provided there are no other caveats which are difficult to satisfy).

There are even theories about Late-Forming dark matter, which predict that dark matter formed after the epoch of Big Bang nucleosynthesis, but before the period of the decoupling of the microwave background.

For similar reasons, to answer the question "how constant have the levels of dark matter remained?" you'll need to consider the model you're using. Scientific communities are presently leaning towards the WIMP model, and hence the theories of dark matter annihilation are relevant to this discussion.

According to our definitions, WIMPs do not have electrical charge; WIMPs are their own anti-particles, so a process of 2 WIMPs annihilating to yield a fermion and its antiparticle (or even a gague boson pair of $W^+$ and $W^-$ or 2 $Z^0$ particles) can take place without violating quantum number conservation. Indeed, this is probably the process through which WIMPs formed in the early universe, with the abundant energy needed for the high mass.

One proposed theory about how this process is involved in the changing quantities of DM involves the process of DM annihilation affecting the evolution of a star, and is described in this paper on arxiv.

Salati, Pierre. “Dark Matter Annihilation in the Universe.” International Journal of Modern Physics: Conference Series, vol. 30, 2014, p. 1460256., doi:10.1142/s2010194514602567.

Paraphrased and simplified, it says that a dark matter halo may collapse into dense regions of WIMPs in the center of a galaxy. Stars nearby could attract some of these WIMPs; the process of annihilation of pairs of WIMPs subsequently takes place and the star evolves into a red giant.

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  • $\begingroup$ To paraphrase the linked answer and your analysis, any dark matter of much more than 10 MeV would have to have origins in the Big Bang or its immediate aftermath. Correct? (FWIW, astronomy data strongly favors keV warm dark matter over heavier WIMP cold dark matter, so the the threshold may not matter much anyway, however.) $\endgroup$ – ohwilleke Jul 31 '18 at 6:45
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    $\begingroup$ @ohwilleke Kind of... it's possible to achieve some energies in particle colliders, but that'll result in very small quantities of the required particle, if any. There're no known processes which can generate a particle of significantly high rest mass regularly and in large quantities. The high temperatures and density during the time around BBN would be sufficient to create relatively humongous quantities. We're also neglecting the significance of the possible necessity to conserve certain quantities other than energy. $\endgroup$ – user191954 Jul 31 '18 at 8:46
  • $\begingroup$ No arguments with the formation part of your answer, but the question also asks whether the dark matter content is constant. It could decay or be annihilated. arxiv.org/abs/1804.01055 $\endgroup$ – Rob Jeffries Jul 31 '18 at 11:00
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    $\begingroup$ @RobJeffries Thanks, I had missed that part of the question. I'm afraid I don't really have the background to confidently write about the contents of that paper you linked, but I have included what I know about dark matter annihilation in WIMPs. I haven't read about theories involving dark matter decay, so I didn't mention that. $\endgroup$ – user191954 Jul 31 '18 at 13:38