Black holes are so massive that not even light can escape their gravitational pull, so by their nature they are invisible. This is the famous reason that we know, but it needs some prefix. And also it is not the only one. The other could relate to its structure.
If we look at the star, plant and also an atom structure, we could find the structure for the black hole too. Black holes have a surrounding area and the more compact center.
We can conclude that in a black hole there is a compact set of protons and neutrons that it creates such a high density and taking into account that a black hole has a surrounding area and this area can be oceans of protons and neutrons. One can also imagine the black hole as an extremely large atom whose nucleus is made of protons and neutrons and an ocean of protons and neutrons and electrons revolving around it.
Universe is expanding at a constant linear velocity and over time the radius of the Universe, simply put, rotational radius, increases. So, in fact, our rotational velocity, which is affected by both the angular velocity parameter and the rotational radius, increases without any force being applied to it. In this paper we have justified the sphericity and the rotation of the Universe by Hubble's law, and we have Calculated the real radius of the Universe and its actual rotational velocity by Hubble's law.