Things are neither that simple nor what you would expect.

There are many shapes of galaxies from dwarf to elliptical to diffuse and other spirals.

They have one thing in common, they revolve around a central core dense in matter. (There are, too, exceptions to this rule.)

They may or may not have one or more black holes and these black holes do not have to be at the center of the galaxy's rotation.

We often have a schematic view of galaxies when they have very varied characteristics.

What makes them look alike is the halo of dark matter that envelops them, it can be very important: 90% of the attraction of the galaxy or, more exceptionally weak.

In conclusion, the hyper massive black hole which can be found at the heart of large galaxies are infinitely too little massive to have the role of "pivot" of the galaxy which shelters them. The mass of the Milky Way is made up of 200 billion solar masses and at least the equivalent of 1000 billion solar masses in dark matter. imagine the little fly represented by its "central black hole" of 4 million solar masses ...

The greatest mystery is that, once these observations have been made, we are astonished to note that our observations of the deep sky show us the first generations of galaxies having black holes disproportionate with the size of these galaxies, and especially, whose formation does not can be explained by the mode of formation of hyper massive black holes observed today.

If you are talking about a spiral galaxy, like our summer or andromeda way, the center of these is so concentrated in matter that a black hole is formed. It is also around the latter that the arms of our galaxy revolve, like the solar system, in much larger and massive recess. As for the nebulas, it varies from their composition. (Tell me if I said stupid things, that's my first answer and I'm only 16)

At the center of our galaxy for example, many scientific theories explain that there would be the presence of a supermassive black hole of about three to four million solar masses.

It is around this that the arms of our galaxy are formed and that we currently revolve. It is still theoretical because it is difficult to observe it, in fact our galaxy is "flat" and as we are on the edge of this one, the numerous gases between our planet and this black hole prevent us from making observations and net calculations:

There is a second theory which explains that it could be a wormhole. Even if their existence has not been proven, mathematically they could exist because they follow all of Einstein's laws.

These spatio-temporal passages to another universe may seem unreal, and yet the physicist Dejan Stojkovic explains that he will be able to prove within a few years the presence of a wormhole at the center of our galaxy: By measuring the orbit of stars revolving around. Indeed, if it's a wormhole, stars from the other universe are probably orbiting on the other side. And their gravitational field would flee through this passage to our reality which would disrupt the orbit of stars in our own universe.

However, we will have to wait because the current measurement tools are not yet efficient enough.