"what is black hole?"
Some posts are clearly not from adults informed about basic cosmology. This simple post implies no understanding to support even a specific question.
my comment to that post:
The most common use for a black hole is an X-ray source with no visible object.
Nearly every galaxy has an X-ray source but nearly all galactic cores are congested with dust, gas, and numerous stars so the source is usually obscured. M87 had an image in radio.
In this case a black hole with an extremely hot accretion disk is proposed.
There is one verified mechanism for generating X-rays: a synchrotron.
Excerpt from the European Synchrotron Radiation Facility site:
Synchrotron radiation was seen for the first time at General Electric in the United States in 1947 in a different type of particle accelerator (synchrotron). It was first considered a nuisance because it caused the particles to lose energy, but it was then recognised in the 1960s as light with exceptional properties that overcame the shortcomings of X-ray tubes.
In the mid- to late 1970s, scientists began to discuss ideas for using synchrotrons to produce extremely bright X-rays.
The entire world of synchrotron science depends on one physical phenomenon: When a moving electron changes direction, it emits energy. When the electron is moving fast enough, the emitted energy is at X-ray wavelength.
This simply defined mechanism for X-rays has been known for roughly 50 years.
Doctors and hospitals have X-ray machines (no black hole).
Modern cosmology ignores this known physics and instead proposes a new mechanism never duplicated.
The theory is a black hole (an unverified theory of a curved coordinate system applying to the cosmos) can cause a surrounding disk of material moving with friction to heat to such an extreme temperature that its thermal radiation extends to X-ray wavelengths.
This mechanism has never been duplicated.
from a post at the University of Cambridge Institute of Astronomy, about thermal emission:
To be hot enough for the peak of emission to be in the X-ray range the material would have a temperature of around 300,000-300,000,000K.
This is absolutely unbelievable for material in an accretion disk (not fully compressed but loose enough so friction alone causes this heat) to reach this extreme temperature and remain intact.
Every X-ray source in the universe is synchrotron radiation, never thermal radiation from a hot accretion disk.
Every galaxy has at its core an electric current bending in a magnetic field.
M87 has a plasmoid at its core. A plasmoid has a torus shape for its electric current bound by its magnetic field. Plasmoids were discovered and named by Bostwick in the 1950's.