Black holes as everyone knows are highly massive objects. All black holes do not form a galaxy. All galaxies have many black holes in it. However for the black hole to be at the center of galaxy it should be highly massive. Millions of solar masses (million times mass of sun). This is because it is believed all stars in the galaxy revolve around this supermassive black hole at the centre of galaxy. Hence stars at large distances need to be bound to the center by a strong force. Hence the center of galaxies can contain only supermassive black holes.
The black hole at the center of galaxy has no proof in the form of image. This is because due to its black colour they cant be seen in the sky. However black holes can be detected by indirect methods.
All black holes have a disc of gaseous materials around it in motion. The temperature of this disc is very high due to high gravitational friction of black hole. Hence at these temperatures the disc glows. We know at high temperatures gases emit radiation(this is the principle of many light sources like mercury vapour lamp).
However they don’t emit in the visible region.
the emitted radiation is in the form of XRay or Radio or sometimes infrared. Hence we can detect the black holes by Xray detecting machines(the principle of these machines is spectroscopy as the wavelength of radiation can be detected by materials that are excited in Xray region).
One such effort by us is the CHANDRA xray observatory that rotates around the Earth. The Chandra observatory has confirmed all these observations at the center of galaxy and astronomers are almost 100% sure that there is a supermassive black hole at the centre of our galaxy.
This black hole is named as Sagittarius A*.