Strange

Pretty much any time we say something like "we know there is a black hole at the centre of the galaxy" or similar statement, what we really mean is "as far as we can tell, all the observations of the centre of the galaxy are consistent, within certain error bounds, with the idea of there being a black hole; this may change as further information is obtained." As this is a science forum, I think those disclaimers can be taken for granted.

So what. We were talking of active black holes with large accretion disks. And then you started talking about the mass of matter around an inactive black hole. The text you quoted explains why. Read it.

That is the main point of the balloon analogy. Or the surface of the Earth: finite but with no boundary and nothing beyond it.

But we were talking of active black holes with large accretion disks.

What do you mean "created alone"? You mean, is there just one? Who knows. Why is that absurd? Why is either yes or no absurd?

Why?

If it has an accretion disk, it is not inactive. What is your source for that? The Milky Way is the obvious example. Then you need to update your understanding: https://en.wikipedia.org/wiki/Sagittarius_A*#Discovery_of_G2_gas_cloud_on_an_accretion_course Also, my understanding is that the stars in orbit around the black hole are not necessarily stable and, sooner or later, one may fall into the black hole. That would be pretty spectacular.

Only in as much as we don't know the how or why of anything. But in fact we have a theory that explains gravity extremely well (General Relativity) and the same theory explains the expansion of space. Here is a good overview: http://math.ucr.edu/home/baez/einstein/ That's about right. The distance between things increases. This is not apparent locally because things are held together by gravity and/or electromagnetic forces. How can "distance" leak? In the analogy, we are only concerned with 2D observers on the 2D surface. In the analogy there is no 3D balloon; there is no inside (or outside). We have no way of knowing if the universe is finite or infinite so, I suppose you can believe whichever pleases you (there seem to be about as many people who believe it must be infinite). But the only scientific answer is "we don't know".

You can ask. Now, about these experiments that show relativity wrong: how about some examples? Why the reluctance?

It also doesn't include acceleration but you are using that in your scenario. If you are going to include acceleration then you have to also include all the other "real world" factors. Like, what is the actual force at different times. You have raised a potentially interesting question but seem to be refusing to explore possible resolutions to the apparent problem. Are you?

Hard to say; that was one very long and convoluted sentences there! You are correct that there are stars and galaxies beyond those we can see. The limited range we can see is called the "observable universe". Beyond that is the rest of the universe, which may just be very large or it may go on for ever. The balloon analogy is flawed in many ways, which as you say, implies physicality. It also uses a 2D surface as an analogy for 3D space, which a lot of people struggle with. It also implies a finite universe. So, one important thing to note is that the "observable universe" in the balloon analogy is a circle around "our" galaxy containing all the galaxies that we can see. There are further galaxies beyond that. (Every galaxy has its own observable universe, and they all overlap.) The other useful point the balloon analogy makes is that the universe could be finite but have no edge: the surface of the balloon has a finite area but there is no edge to it. Finally, all these descriptions ("expanding space", space growing", "space being created", "distances increasing", etc) are just different metaphors to describe what the theory (i.e. the mathematics) describes. They all have the potential to confuse or mislead in slightly different ways!

g = 9.8 m/s2

I'm not sure what that means. When you say "orbits itself", do you just mean "it rotates"? This is not related to the Earth's gravity, as far as I know. It does. It rotates once in 27 days. The same time it takes to orbit the Earth. (See Tidal Locking link above.)

More discussion of the various issues here: Court fight over Oxford commas and asyndetic lists

I don't think there is any real difference between those descriptions. Space isn't "stuff" so it isn't created in that sense. But expansion means that things move apart (on large enough scales) and so there is more space between them.

I have no idea. But apparently, the accretion disk can be HUGE. http://astronomy.swin.edu.au/cosmos/A/Active+Galactic+Nuclei I guess it varies depending on the size of the black hole, the amount of material in the accretion disk, etc. And then they become quiet, until they encounter another gas cloud, or capture another nearby star. https://phys.org/news/2016-11-galactic-nuclei.html There is a limit to how much matter can fall into a black hole. A certain amount is "blown away" by the heat and radiation produced by inflating matter. I think this is one of the challenges of explaining supermassive black holes: they cannot absorb enough material to grow as large as they do.

So where is the experiment that refutes the principle of relativity? And why refer to Poincare? Did he say anything different from Lorentz or Einstein or ...

This is symptomatic of most of what you post. You jump to implausible and supportable conclusions based largely on a lack of knowledge. But you are correct, we haven't observed black holes directly (yet). We have only observed the effects of the presence of a black hole. These can really only be explained by the presence of black holes. There is a project to directly view the black hole at the centre of our galaxy. http://www.eventhorizontelescope.org How can you see something black? The same way you can see shadows or eclipses.

The light is both too faint and it has been shifted out of the visible range. There are only a finite number of stars and galaxies that can be seen. The amount of light from them falls off, roughly speaking, with the square of the distance (a galaxy 4 times as far away will be 16 times fainter). I say roughly, because there are other factors such as the type and age of the galaxy, intervening gas and dust clouds, etc. Note that most of this light is not reflected but is generated by stars like our sun. The only objects (visible to the naked eye) that reflect light are the moon and 5 planets.

No theory needs black holes. We just happen to observe them.

No it isn't.

I'm not sure it was a compliment!

"Nearly"

Even though I said "based on observation" ? How much more physical than that can you get. What science doesn't do is deal in metaphysics (e.g. "what is reality"); that is a question for religion or philosophy. By definition it is not empirically testable.

That sounds like a very crude version of a pseudo random number generator. This is not true. Sometimes people are recommended to seed the RNG using he clock, but this doesn't improve randomness at all.