The Black Hole at the center of the Galaxy

Milky Way is a spiral galaxy, it is formed by clouds of gas, cosmic dust, dark matter, energy, planets and stars all these structures are linked by the existence of gravity and they revolve around the center of the galaxy, also The Sun revolves around the center of the galaxy at a speed of 220 Km/s.

If the structures that exist in the galaxy revolve around galactic center then what about the center?

Catch of the accretion disk of Sagittarius A * taken from 

the space simulator spaceengine.

In the center is a supermassive black hole its name is Sagittarius A* (Sgr A*).
Sgr A* has a mass of 4.31*10 (Exponent 6) Solar masses, that is to say, 4 million times the mass of The Sun. it has never been seen (because it doesn't emit radiation in the form of light), but it exist by the mysterious movement of the nearest stars, and of all the Galaxy.

In the Center of the galaxy there are many stars, and emits large amounts of radiation.

The movement of the accretion disk of Sagittarius A * and its stars, along with the black hole recorded from spaceengine looks like this: