Alfredo Tifi said:
I read that
1. dark matter has to be concentrated in galaxies;
2. McGaugh & Co discovered a precise relationship between visible-ordinary matter and the calculated sum of ordinary + dark matter from thorough observation of actual acceleration of more than 150 galaxies.
3. First experiments to reveal effects or interactions of dark matter have failed.
4. Galaxies have giant black holes in their cores.
Before going ahead with Modified-Gravity Theories, why does not anybody take into account the hypothesis that dark matter is nothing else but normal matter in black holes, thus invisible? MacGaugh's relationship could be the result of a correlation between visible mass and black hole dimension. The four points would agree.
1. It would be more accurate to say that DM concentrates
around galaxies ( the majority of dark matter is in regions where we see no visible matter.
2. While there may be a relationship between the relative ratio of dark matter to luminous matter per in galaxies ( though this recent discovery seems to cast doubt on that assertion in for all cases;
https://Earth'sky.org/space/ngc-1052-df2-galaxy-without-dark-matter )
This doesn't mean that the distribution of dark matter mirrors that of the luminous matter. In fact, in order to explain the rotation curves, it can't.
3. This is a case of where lack of evidence, is not evidence of lack. We are looking for something that would only react weakly (if at all) with other matter, with the exception of gravity. Gravity is such a weak force, that you would need a lot of Dark matter to detect it that way. (like when it effect the rotation curves of a galaxy. For years, critics kept claiming that the failure to detect Gravitational waves was evidence of their non-existence, but we have now detected them.
4. Don't be misled by the "supermassive" " in supermassive black hole", this is relative term used to compare them to stellar sized BHs. a typical supermassive black hole only has a small fraction of the mass of the total luminous matter in its galaxy. To explain galaxy rotation curves you need amounts in excess of the luminous matter mass.
Trying to squeeze all that extra mass into the central black hole doesn't solve the problem. It would actually cause the predicted rotation curve to deviate even further from what we measure. In order to match what we measure, the extra mass has to be spread out into a spherical volume that the visible galaxy sits in. So for a spiral galaxy like our own, a good deal of it would have to be in the regions above and below the visible disk. If this were to be explained by DM being BH, you would have to explain why all those black holes formed out where there are hardly any stars.
Another problem has to do with the fact that those black holes would have had been formed, as you put it, from "normal matter". But or models of the formation of the universe preclude that. It goes back to the early universe and to when the elements first started forming. If the universe had very much more baryonic matter in it that we see now, this process would have been effected. The result we would see today would be a different relative distribution of elements. In other words, the fact that universe is now made up of 74% hydrogen, 24% Helium, 10.4% oxygen, and so on, means that the universe can't have too much matter hiding in black holes. The limit of how much matter could be hiding in Black holes is much less that needed to account for the effects attributed to dark matter.
The point is, astronomers are not going to pick a complicated answer over a simple one if there is a chance of the simple one being right. They'll look for simple solutions first, and move on to more complicated ones if they don't pan out. So don't assume that they "overlooked" the simple answer. It is much more likely that is was considered and then rejected for failing to pass muster.