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.
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.
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