Why is there a super-massive black hole thought to be in the center of each Galaxy?
There is known to be a supermassive black hole called Sagittarius A* at the centre of our Milky Way galaxy and other nearby large galaxies.
The theory is now that most large galaxies have a supermassive black hole at their centre. There is also a relationship between the mass of the supermassive black hole and the mass of the galaxy's central bulge.
There is also evidence that emissions from the supermassive black hole affects star formation.
It is possible for a galaxy to lose its supermassive black hole. If two galaxies collide the two supermassive black holes can merge leaving one galaxy without one.