It is assumed that the density of particles in the empty space between stars or galaxies is extremely low. But how do we know this density? The space surrounding the Sun is contaminated by cosmic radiation. Would a different value of this density alter what we see?

If that density were 10 times as large as expected could it account for dark matter? A galaxy contains a lot of mass but also a huge volume. If there is a higher density as expected wouldn't that make its gravity pull 10 times as strong? How big is the pull with the current density?


1 Answer 1


The density of material in the interstellar medium is inferred from (i) the electromagnetic radiation it emits; (ii) its effect on electromagnetic radiation passing through it. Often these approaches are combined to learn about different "phases" of the interstellar medium - e.g. hot, cold, high, or low density, ionised or not.

For example, we can learn about how much atomic hydrogen there is using 21 cm wavelength radio emission. Molecular gas can be traced by molecular emission lines in the infrared. X-ray, and particularly EUV, emission is absorbed by the interstellar medium. UV and optical absorption lines can be seen in the otherwise featureless continuum spectra of hot stars. The spectra of stars are reddened by dust scattering, etc.

All these techniques that probe the interstellar material rely on interactions with electromagnetic radiation. So technically, none of the matter probed in this way is dark matter and the technical answer to your question is no. Most of the dark matter in the universe does not interact electromagnetically and the density of such matter is inferred solely by monitoring it's gravitational influence.

I suppose what you are asking, is could there be ten times as much "cold" interstellar medium as we thought and could this account for the rotation curve of our Galaxy?

The answer to that is also no. Even cold dust or gas is not cold enough to emit no radiation - we would observe it in the far infrared and microwaves. Equally, a factor of 10 just wouldn't do it. The interstellar medium forms a small fraction of the total mass in the Galactic disc (about 30% - Chabrier 2001) and thus only 2-3% of the total inferred Galactic mass; and it is highly concentrated towards the Galactic plane and centre. Explaining the dynamics of the Galaxy requires a spherical distribution of dark matter that extends well beyond where the bulk of visible material is found. Any spherical distribution of matter in the form of gas or dust would have collapsed into a disc long ago.

There is also a broader cosmological reason why the answer to this question (and all variants of it) is no. As mentioned above, we know, from a combination of careful measurements of the cosmic microwave background and estimates of the primordial abundances of helium and deuterium, that baryonic matter makes up only 20% of the gravitating matter. Thus the only "particles" that could be present in an abundance sufficient to explain the dark matter problem are non-baryonic. If we take the halo of our Galaxy as an example, the fact that we see no evidence of the emission or absorption of light by the huge amount of mass that is dynamically inferred, fits perfectly with the explanation of non-baryonic dark matter. i.e. The only abundant particles that can account for the dynamics of our Galaxy are non-baryonic dark matter particles.

  • $\begingroup$ What about the truly empty space? If we look at an ordinary galaxy. Couldn't there be five times as many particles as the number of particles in the main ingredients of the galaxy (stars) contained in the whole volume of the galaxy? There are (rough calculation) 10exp11 times 10exp23 times 10exp15 particles. That is 10exp48. The volume of the galaxy is about 10exp48 too. Cant there be 5 particles per cube meter? So there is 5 times as many matter in empty space as in the stars? Or are 5 particles already too much for empty space? $\endgroup$ Commented Jul 31, 2021 at 9:57
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    $\begingroup$ To add: There are several observations that depict non-baryonic DM is much greater than the baryonic mass. Perhaps the two strongest are the CMB and structure formation. The other issue is that we have a census of the majority of the baryons in space that make up the cosmic baryon fraction so it's unlikely there is extra mass in the form of particles elsewhere that are baryonic. Especially to be significant enough to be DM. Dynamics suggest DM resides in a spherical halo around galaxies. What would be the physical mechanism for "empty space" to have this specific distribution? $\endgroup$
    – Astroturf
    Commented Jul 31, 2021 at 12:42

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