From what I gather the interstellar medium has about about 1 atom per cubic centimeter. But on the other hand, as they say, "Space is big, really really big" So if it is known (or at least theorized about) what percent of the universes matter is in the following: black holes, stars, and interstellar medium?


There is much more matter in the interstellar medium than in the visible stars.

Our best estimate of the total matter/energy content of the universe is shown in this image from NASA: Total Matter/Energy Content of the Universe

Now the Dark Energy is not really matter - it acts like an anti-gravitational vacuum energy which is responsible for the accelerating expansion of the universe. The Dark Matter is probably some kind of matter that does not have any strong nuclear or electromagnetic interactions with our ordinary (baryonic) matter (Atoms). It may possibly only have gravitational interactions, but all the current searches for Dark Matter assume that it also has a weak nuclear interaction with ordinary matter.

So that leaves $4.6\%$ of the universe that is made from atoms. It is believed that only about $\frac{1}{2}\%$ of the mass of the universe is in the form of the atoms that are inside stars (and planets). So for every atom in a star, there are 9 atoms in the interstellar medium. Some of the atoms in the interstellar medium would be gas clouds inside of galaxies and some of it would be the diffuse interstellar gas between galaxies and even clusters of galaxies.

The amount of mass/energy in black holes is a more difficult question (and about which I know much less). Some of the atoms have already collapsed into black holes such as the super-massive black holes in the center of most galaxies and the black holes that may result when stars go supernova. The super-massive black holes in the centers of galaxies are still only a very small fraction of the total mass of the galaxy so all of these "known" black holes are probably only a small fraction of the $\frac{1}{2}\%$ of the atoms that are in the form of stars.

However, there could be some primordial black holes left over from the Big Bang. These primordial black holes could constitute part of the Dark Matter of the universe. Most standard cosmological theories don't predict any (or many) primordial black holes and there have been some searches for them with gravitational lensing. In this area, I know much less, but I believe these primordial black holes cannot be a big fraction of the Dark Matter, but I do not know what the upper limit is precisely.

  • $\begingroup$ +1 Good, thorough answer. If the OP was referring to the ISM the same way astronomers do, then I think the key component of the answer is that about 1/10th of gas in a galaxy is in stars, of which very little will be in compact remnants. So there is more mass in interstellar material than in stars. $\endgroup$ – Warrick Dec 9 '11 at 9:00
  • $\begingroup$ Great answer! Thank you, and sorry I didn't see your answer for a few days. I thought the answer was going to be more matter in stars by a long shot. $\endgroup$ – Joe Dec 14 '11 at 20:10

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