What fraction of the mass of all baryonic matter are black holes?

Question

Is there a calculation of how much of the mass of baryonic matter is black holes, including supermassive black holes in the center of galaxies?

Answer 1

The mass that is known to exist within (non-primordial) black holes is much less than that known to exist in stars.

Even the biggest supermassive black holes are typically 1-10% of the stellar mass of the galactic bulges they reside in, and more typically 0.5% (e.g. see the M-sigma relation).

Only the most massive stars produce "stellar mass" black holes - so this is a tiny fraction of the stellar population and total stellar mass (<1%) because the birth stellar mass distribution is heavily biased towards low-mass stars ($N(M) \propto M^{-2.3}$).

In addition, microlensing searches have failed to find any evidence for large populations of massive compact halo objects.

On top of this, it has been estimated that even the mass of baryons betrayed by their presence in stars is only of order 10% of the total baryonic mass; with most of it being spread between galaxies in the form of ionised hydrogen from the big bang (see What fraction of baryonic matter is in stars?).

So if pushed for an estimate I would say that supermassive black holes plus all the stellar-sized black holes add up to $\sim 1$% of the stellar mass and that this in turn is $\sim 0.1$% of the baryonic mass.

A more careful calculation by Fukugita & Peebles (2004), pretty much agrees with this. They say stellar and supermassive black holes account for about 0.008% of the critical density (with an uncertainty of about a factor of 2), which is 0.16% of the baryonic density.