There are currently two (mutually exclusive) theories regarding the formation of galaxies, which according to current models, all have a supermassive Black Hole at their centre.

In brief, the theories are that :

1) The galaxy formed around a pre-existing Black Hole which was sufficiently massive as to attract all the nearby stars.

2) The galaxy formed first, and the Black Hole then formed by the merging of stars at its centre.

Further information. In Out Time , BBC audio discussion.

Also see :Galaxies (definition of)plugin-autotooltip__plain plugin-autotooltip_bigGalaxies (definition of)

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Although current astronomical techniques have revealed the presence of billions of entire galaxies, stretching to the 'edge' of the known universe, a strict definition of what a galaxy actually is has not been generally agreed.

" and Gravitational singularitiesplugin-autotooltip__plain plugin-autotooltip_bigGravitational singularities

"A gravitational singularity, spacetime singularity or simply singularity is a location in spacetime where the mass and gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system.