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The observable matter in the universe is distributed and arranged into various structures, including black holes, stars, nebulae, and the much more diffuse regions of the inter-stellar and inter-galactic mediums.

Q. How much observable matter, as a percentage of total mass of observable matter in the universe, is arranged into black holes, stars, nebulae, etc. respectively?

This seems like a pretty basic question, but Googling has not turned up satisfactory results (instead, it tells me that e.g. observable matter is only 5% of the total mass-energy of the universe, which is not what I asked for).

I would expect that something like 99% of observable matter is trapped within stars, with only <1% being distributed throughout the interstellar medium (e.g. in nebulae). I would also expect that the total amount of mass inside black holes would be orders of magnitude lower than that trapped within stars. Are these estimations actually correct and backed up by evidence?

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  • $\begingroup$ Is "observable matter" all matter apart from dark matter? Eg, does it include particles whose energy is too low to detect directly, like most neutrinos, especially the Cosmic Neutrino Background? BTW, isolated black holes that aren't absorbing matter are very hard to detect, so they aren't any more detectable than dark matter. $\endgroup$
    – PM 2Ring
    Commented Feb 28, 2022 at 2:06
  • $\begingroup$ @PM2Ring Good question, one that I hadn't thought of. I was indeed using "observable matter" to refer to any matter except dark matter, but I see there might be an issue. On your second point, I would think that, even if individual black holes without accretion disks are hard to detect, there might be methods to come up with estimates about their overall density in the universe? Though I may be wrong about this. $\endgroup$
    – YiFan
    Commented Feb 28, 2022 at 2:14
  • $\begingroup$ Yes, we can estimate black hole numbers, from models of star formation. Not many stars form black holes, and the life span of those stars is relatively short. $\endgroup$
    – PM 2Ring
    Commented Feb 28, 2022 at 2:19
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    $\begingroup$ IIRC, despite it's extremely low density, there's quite a lot of matter in the IGM (intergalactic medium). I'm sure we have a good answer about that, but I can't find it. (Maybe it's on Physics.SE). $\endgroup$
    – PM 2Ring
    Commented Feb 28, 2022 at 2:21
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    $\begingroup$ +1 for asking a question that I didn't know I wanted the answer to, and that actually has an estimated answer. $\endgroup$ Commented Mar 2, 2022 at 19:52

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I think the following is a fair summary. It's based on a fairly old study by Fukugita & Peebles (2004) but the numbers are quite reasonable. Your guess about stars dominating is way off. Most of the baryonic mass is in the intergalactic medium. However it's likely that black holes are a very small fraction of the stellar mass. A remaining uncertainty is whether primordial black holes form part of the dark matter.

The plot was produced by Markus Possel.

Inventory of the universe

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  • $\begingroup$ Nice, thank you! Seems like my intuition is completely thrown off when it comes to things of such a large scale. I also didn't realise the CMBR and neutrino background were such a significant contribution. $\endgroup$
    – YiFan
    Commented Mar 1, 2022 at 23:57

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