I think IFN use for astrophotography seems the best choice. It is a description of what makes it visible to our cameras, even professional's cameras. The collective light from the bulk galactic starfield makes it so. While cirrus may be what astronomers use, the origins of that word come from meteorologists term for high thin weather clouds in our atmosphere. It therefore is used through a logical borrow. My guess is that cirrus has been around longer since the pros have been able to see IFN with their large telescopes and superior cameras and locations. But just because cirrus may have been the first (though logical name given at the time) doesn't mean it is better or more "professional". As a scientist (in a different field) I have seen many naming conventions come about simply because one person, often the first person to see a phenomenon, comes up with a name at a meeting or first publication that sticks. In fact, I have seen IFN used in more recent professional astronomical publications, and I would not be surprised that it overtakes cirrus. IFN is catchy, it is more accurately descriptive of the clouds location (after all, the dust clouds must be removed from the galaxy center if it is to be reflecting collective light from the galaxy) and the mechanism that allows it to be seen and is unbiased as to cloud density or morphology. In fact I have seen images on Astrobin and elsewhere that show a close association of IFN that contains some areas of reflection nebula and/or even some quite dense regions.
@Wei-Hao Wang is correct that the vast majority of the materials in molecular clouds is hydrogen, followed by helium, etc (the primordial gases). But the optical densities of molecular clouds mean that they must contain larger particles than just atoms or diatomic or other simple molecules, because these are transparent to visible light. Also, if the clouds were to consist of only H and He, they would not be able to achieve the extremely cold temperatures required for the condensation into stars. So there are higher order molecules, such as organic (carbon rich), (even buckminsterfullerine, quite large and clumps together readily into soot!), some of which are large enough to condense into actual physical particles, like soot that can absorb and reflect visible light. These clouds also must have all the materials that go into the construction of planets, including water, silicon, and the lighter metals up to iron. And most rarely, materials heavier than iron. IFN
must have some actual physical particles. It is only particles and larger molecules that can reflect, absorb and re-emit the galaxy light. If it was only gas, the light that we record would just pass right through. Since molecular clouds have atoms heavier that lithium, and particles and agglomerates of the heavier materials, the only way this material could have arisen is from supernovae and as we learned in the last couple years, the merger of neutron stars (which is now thought likely to be the only way elements heavier that iron can be wrought!).
The current theories of nucleosynthesis becomes a real challenge to theoreticians, because at the earliest age of the universe, just as stars were thought to be lighting up, we know that only H, He and Li were present. So therefore, "all" of the dust that exists in all galaxies must be the result of the gradual collection of star death debris over 14 billion years. The impact that has on star formation is critical. The first stars must have formed through very different mechanisms, because there were no molecular clouds at the beginning and currently we really only see new stars being formed in molecular clouds. Molecular clouds only work because they can become much colder than the non-cloud expanses of interstellar or intergalactic space. Just clouds of H and He cannot now form stars. They stay much too hot and the atoms cannot condense. Dust gets cold, because when dust particles collide, the particles lose kinetic energy, often by emitting long wavelength light such as long wave IR and get colder. Often very close to absolute zero. You might ask, would the IR not just be reabsorbed by the dust grains? Fact is IR passes through dust and escapes the clouds, that is why astronomers can use IR, microwaves and radio waves to penetrate clouds to see things deep within. And the dust cloud particles block the shorter wavelength light, which defends the cloud against energy penetrating deep within, where it gets coldest. So currently, it is the dust that drives the formation of stars, even though H is the more abundant. Really, H and He just go along for the ride. So the question is, how did the first stars ever form? Multiple theories on that! And they are changing. Interesting to read, and the mechanism for that was unique to the epoch and conditions back then. And the stars were much much bigger.
