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 the interstellar material probed by these techniques relies on an interaction with electromagnetic radiation. So technically, none of it 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.

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.

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 the interstellar material probed by these techniques relies on an interaction with electromagnetic radiation. So technically, none of it 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 very small fraction of the total mass of the Galaxy and is 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.

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 the interstellar material probed by these techniques relies on an interaction with electromagnetic radiation. So technically, none of it 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.