http://www.newsweek.com/milky-way-supermassive-black-hole-collision-large-magellanic-cloud-1278883

A “great collision” between the Milky Way and another nearby galaxy could result in the awakening of a suppermassive black hole, causing it to swell up to eight times its current mass, devouring surrounding gas and throwing out huge amounts of high-energy radiation.

Galaxies are not stagnant. They are moving around space all the time and often collide and merge. It is thought that in around eight billion years, the Milky Way will collide with Andromeda, our closest neighbouring galaxy.