"An Engine of Destruction..."

Neat little article

At the center of the Milky Way galaxy lies Sagittarius A-star, a supermassive black hole, which undoes any lovely notions of light or creation: because our entire galaxy is held together by an engine of destruction.

A sixteen year study by scientists at the Max Planck Institute tracked the orbits of central stars orbiting the black hole.  Yes, sixteen years, and that's the very smallest of the dimensions involved here {...}  Of all the places in the universe, a galactic core is the most likely place for a vast star to form (and then collapse into a black hole), or for a smaller black hole to consume enough matter to become supermassive.

Once the hole passes a certain size its gravitational attraction will shape the orbits of everything it doesn't eat.  It no longer matters if it was exactly at the center before - it's made itself the center now  It seems inevitable: all black holes below a certain size evaporate, and above a certain size they just keep growing.  Wait long enough and one will turn up.  And eat you.  {...}

Galaxy 0402+379, which is kind of an unimposing name for the greatest unexploded bomb in existence [is a special case].  It doesn't have one supermassive black hole - it has two, and is thought to be the result of a truly massive collision between two galaxies, each with only the standard "one mega-ultra-huge black hole per galaxy".  A galactic collision is the second most amazingly violent event you can think of - the first will be when those two black holes eventually hit each other.  The resulting merger will release energy on an utterly unprecedented scale, and emit gravity waves which will bend spacetime itself.