Inspired by this question about whether dark matter is matter, noting that dark matter tends to be clumped in galaxies near the center and less so on the edges, accepting that many (most?) galaxies have large black holes at their center, and theoretically, black holes have 'infinite' gravity, could black holes actually be the dark matter that holds galaxies together?

Put another way, how do we model the 'infinite' gravity of a black hole when considering the dynamics of a galaxy?

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    Black holes don't have infinite gravity. Besides, the equations formulated for the rotation of the galaxies did not at all match with the force they should have possessed while turning, black holes at the center included. – Curious Fish Oct 22 at 19:19
  • You're citation above "tends to be clumped" actually explains that dark matter is just MORE concentrated at galactic centers, not exclusively. "Chicken and egg": the distribution of dark matter in the universe probably determined the distribution of baryonic matter. A better question would be "Can dark matter form gravitationally bound objects?" The answer to that would be "No.", because it would require "electrostatic collisions" that dark matter doesn't appear to do. – Jack R. Woods Nov 12 at 16:52
up vote 20 down vote accepted

Do the black holes at the center of galaxies account for the experimental results that prompted the introduction of dark matter? No

The primary piece of evidence that originally sparked the idea of dark matter is the rotation curve of galaxies. We found that galaxies don't rotate like the luminous matter suggests it should rotate. Specifically, given some estimates of enclosed mass at some radius from the center of a given galaxy, galaxies were found to rotate faster at given radii than expected. In other words, the luminous matter didn't seem to account for all the mass within any given radius. This lead to the idea that galaxies are permeated by a "dark matter" that isn't luminous. For this idea to work though, the dark matter needs to permeate the galaxy, it can't all be concentrated at the center of the galaxy like the central black hole (to really explain why this is one would have to get into a bit more details into rotation curves and how the expectations differ from observations).

Other pieces of evidence, like the dynamics which occurs when two galactic clusters collide also wouldn't be accounted for by galactic central black holes. See e.g. the bullet cluster.

Is it possible that the dark matter is made up of many smaller black holes? Possible, but not likely.

At one point in time, there was conjecture that the dark matter consisted of (moderate sized) black holes and other compact objects which have low luminosity. This was the MACHO theory (MAssive Compact Halo Objects). But this theory has largely fallen out of favor.

As Ben points out in a comment, another candidate might be primordial black holes, but their abundance appears to be too low to be good candidates at this time.

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    The debate over MACHOs vs. WIMPs (weakly interacting massive particles) as dark matter has got to be one of the more amusingly named debates in the history of physics. – Michael Seifert Oct 22 at 21:18
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    Especially as WIMPs are currently winning... – Jonathan Twite Oct 23 at 7:46
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    It's like they say: the meek will inherit the Earth, but the WIMPs will control the universe. – Nathan Chere Oct 23 at 8:28
  • It's not attributable to red-shift, identified in the spectrum but overlooked in the dynamics, is it? – elliot svensson Oct 23 at 16:16

Black holes don't have "infinite gravity". The only "infinite thing" associated with a black hole is that the value of the Riemann curvature tensor at the center of a (Schwarzschild or Reissner-Nordstrom) black hole is infinity according to general relativity. But we know that general relativity is not supposed to be a reliable theory near the center of the black hole (where the quantum gravity effects presumably take over)--and thus, even this single infinitely large quantity that we can associate with a black hole is also not really reliably infinite.

Anyway, even if the curvature were infinitely large at the center of the black hole, the effects of gravitation of the black hole are always finite at every point (inside and outside of the black hole) except for the center itself. In fact, according to the Birkhoff theorem (for spherically symmetric stationary black holes), the gravitational effects outside the event horizon cannot be identified as any different from the same of an equally massive spherically symmetric non-black object (which would be centered at the center of the black hole)! So, for example, if the Sun collapsed to a black hole, the Earth (or Mercury or anything outside the surface of the Sun) wouldn't feel a thing as far as the gravitational effects are concerned. So, there is no such thing as "an infinite gravity of a black hole".

Now, dark matter is supposed to be distributed throughout the galaxy. There might be some density distribution I suppose but all of dark matter isn't centered at the center of the galaxy. If it were, we wouldn't find anomalies in the galactic rotation curves which led us to postulate dark matter in the first place! So, no, dark matter (at least that we observe throughout the galaxy) cannot be just some effect due to the black hole at the center of the galaxy.

This galaxy rotation simulation from Wikipedia explains pretty well how the concept of the dark matter came into existence. Your assumption that the mass of a galaxy belongs to the baryonic matter and the central black hole corresponds to the left video, where stars closer to the galaxy centre rotate faster than the ones on the periphery. What the astronomers actually observe is similar to the right video, where the orbital speed of the stars doesn't change much, or even slightly increases with distance from the centre of a galaxy.

The observed rotation curves suggest that the total mass of a galaxy is distributed differently from its observable mass, even including the black holes. Furthermore, adjusting the estimated mass of the central black hole changes absolute orbital speeds, not the relationship between orbital speeds and radial distances. Dark matter is a concept which was introduced primarily to account for this discrepancy between observations and simulations, which could not be explained with observable objects alone.

Too answer such questions, we need a model of the entire universe. I believe the universe is closed and cyclic as Albert Einstein proposed. Furthermore, I believe it is finite and that it is finite and that if other universes exist, they do so not in parallel but spatially in series. We as humans have a bad habit of complicating things we do not understand and discovery of particles, subparticles and now strings without a fundamental understanding of the very likelihood that everything is made of a fundamental substance. I believe that 'substance' is space, volume or a reviewing of the archaic term 'Aether'. The latter term seems to conjure up more criticism and Aether has been universally ruled out to exist before further study was able to continue. If my suggestion is true, than matter, volume, 'dark matter' and 'dark energy' are fundamentally the same thing, differing by the modulation, standing wave and density nature. There is the suggestion that black holes are 'massive objects' which I disagree, I do not believe they are objects or that there even have mass. I believe they are regions of extremely compressed space, even more compressed than any matter, matter that falls in compressed of its modulated identity. As for infinite gravity, very unlikely because according to the General Theory, time would cease. How do I ascertain that? Well, it is believed that some binary systems discovered likely consist of a blackhole that is consuming its neighbor star and if it is doing that then how can it orbit if time has ceased within itself. Furthermore, I believe that blackholes, all formed at the beginning over 13B years ago, have a non-existent interior, a void and that the inner edge of space within the blackhole is compressed to where time almost stops, but not completely. An analogy is compressing a coil spring, as you compress it, it becomes even more difficult to compress completely. I have a larger macro view of the universe and this is but part of it. This is not a theory, it is a hypothesis, an idea alone.

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    Thanks James. While your hypothesis is interesting, it needs a thorough fleshing out and vetting according to the scientific method. Though Aether is a non-starter, space is indeed a medium with properties that dictate how matter and energy move through it. There are constants we've associated with this medium much as we've given constants to water, air, glass, etc. as mediums through which light, sound, and matter traverse. I don't believe this is an answer nor a proper frame challenge and thus not currently deserving an up-vote. I suggest you flesh it out more and offer supporting evidence. – CramerTV Oct 24 at 19:23

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