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So I just finished watching "dark matter is not a theory". An understanding I gleaned from it is that dark matter is observed from the discrepancy between the amount (or perhaps energy? in terms of spectrum) of light a galaxy is emitting, and its rotational speed. These are both indications of mass, and they should match up, but the fact that they don't indicates that there is an extra mass there, but one which does not emit light.

Long story short, I have a friend who believes that dark matter is just "regular" dust that is blocking light emitted from the galaxies with observed dark matter.

What are the arguments against dark matter just being "dust", or non-star matter? I know there are a number of "exotic" theories, such as unknown/unpredicted particles types (WIMPs, MaCHOs) and such-- why is a more prosaic solution not preferred over these?

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    $\begingroup$ For most of its history, "dark" matter meant precisely what you say -- ordinary matter that happens to be too dark to see. But people have been looking for such dust for almost a hundred years. (You can tell if it's there by the way it emits and absorbs light, e.g. hydrogen atoms emit radiation of 21 cm wavelength.) They haven't found it. $\endgroup$
    – knzhou
    Commented Mar 12 at 18:39
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    $\begingroup$ @knzhou Your comment seems like it could be an answer, if you expound on it a little? $\endgroup$
    – user151841
    Commented Mar 12 at 18:45
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    $\begingroup$ JWST would see the dust. $\endgroup$
    – cmaster - reinstate monica
    Commented Mar 13 at 10:56
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    $\begingroup$ @cmaster-reinstatemonica how would it "see" the dust? Isn't it dark? $\endgroup$
    – user151841
    Commented Mar 13 at 18:26
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    $\begingroup$ @user151841 Nope. Not in the infrared. JWST is an infrared-only telescope with both excellent light collection and the ability to detect wavelengths as long as $28\mu m$ which means that it can see the thermal glow of any dust with a temperature of about 100K in our cosmic neighborhood. There are beautiful images where the false colors have been chosen to show only the dust present in a galaxy, rather than its starlight. And for the dust to be a dark matter candidate, the dust would need to be within the galaxies where it can be seen by JWST. $\endgroup$
    – cmaster - reinstate monica
    Commented Mar 13 at 18:53

3 Answers 3

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Although this is not how it arose historically, the most precise evidence for nonbaryonic dark matter now comes from the early universe. The is no place for "baryonic dark matter" to hide in the early universe: the temperature and density are high enough that all baryons are strongly coupled, and so all baryonic matter is the same.

Inferences based on early-universe physics are consistent with baryons comprising about 5% of the energy density within the present-day universe and nonbaryonic dark matter comprising about 26%. If the abundance of baryonic matter were much higher than that, then:

  1. The relative abundances of light elements/isotopes that emerged from primordial nucleosynthesis would be very different. For example, the fractional abundance of deuterium would be a lot lower, because with more baryons, there is more time for the deuterium to fuse into helium before the density of baryons drops too low.
  2. Sound waves in the primordial plasma would have much higher amplitudes, since sound can transmit through baryons but not through nonbaryonic dark matter. That would leave a clear imprint in the pattern of temperature fluctuations in the cosmic microwave background.
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    $\begingroup$ Looking at the portion of barionic matter in the early universe is surely intersting, but does it have any hard implications for the barionic matter portion in our present universe? $\endgroup$
    – Zaph
    Commented Mar 12 at 21:30
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    $\begingroup$ To the extent that we understand baryonic physics (and note that the energy scales are sub-MeV, easily accessible in experiments), there's no mechanism to add new baryons to the universe after primordial nucleosynthesis, let alone after last scattering of the cosmic microwave background. It should be noted as well that the cosmic microwave background also separately measures the abundance of total matter, meaning that we know dark matter was present in the early universe irrespective of its abundance today. $\endgroup$
    – Sten
    Commented Mar 12 at 22:02
  • $\begingroup$ So I, take it, there are strong reason to claim that the amount baryonic matter should have not changed. What about the amount of dark-matter? Any reasons to believe that its amount stayed constant threw the development from the early universe to now? $\endgroup$
    – Zaph
    Commented Mar 12 at 22:14
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    $\begingroup$ The amount of dark matter in the early universe matches the amount today, although present-day measurements are less precise, and people do consider models like decaying dark matter that would slightly change its abundance over time. Note that messing with the dark matter abundance over time can also change things like the expansion history and the growth of inhomogeneity (but that's not necessarily bad if you want to solve the Hubble or $\sigma_8$ tensions). Generally it's simplest to assume that the amount of dark matter is fixed, but we can't rule out small changes. $\endgroup$
    – Sten
    Commented Mar 12 at 22:59
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    $\begingroup$ @user151841 Yes, but I should emphasize that the evidence from galactic kinematics (and other late-time effects) is still there and still compelling. It's just that the physics of the early universe are so simple, and the cosmic microwave background so precisely measured, that there is absolutely no way to get out of having nonbaryonic dark matter in the early universe. $\endgroup$
    – Sten
    Commented Mar 13 at 19:19
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Where would all this "dust" come from?

The Big Bang left the universe consisting mostly of hydrogen and a bit of helium. Star formation and evolution formed the heavier elements. Even today, the universe is still mostly hydrogen.

For dark matter to be dust (which has to be made of heavier elements), you'd need six times as much matter in the form of heavy elements as there is hydrogen in stars. The universe simply isn't old enough for that to be the case.

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Your friend's position has the obvious problem that ordinary dust would still radiate, it'd just radiate in infrared wavelengths that human eyes cannot see. But instruments can. If the radiation is not detected, then the obvious implication is that the postulated dust does not exist.

See Wikipedia on baryonic dark matter.

However, multiple lines of evidence suggest the majority of dark matter is not baryonic:

  • Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
  • The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang. Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density. In contrast, large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.
  • Astronomical searches for gravitational microlensing in the Milky Way found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30 solar masses, which covers nearly all the plausible candidates.
  • Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background. Observations by WMAP and Planck indicate that around five-sixths of the total matter is in a form that interacts significantly with ordinary matter or photons only through gravitational effects.

The last point (on CMB anisotropies) is admittedly technical. If you're willing to dive into it, here's an intermediate-level description.

There's also this line of evidence for non-baryonic dark matter which I'm surprised Wiki does not mention in the section on baryonic dark matter:

Structure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the Friedmann solutions to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure. If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.

Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive potential well for ordinary matter collapsing later, speeding up the structure formation process.

The latter half of the first paragraph is the key one. Without dark matter, we would not expect galaxies and clusters.

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