It's the year 4000 (or whatever), and mankind has cheated FTL travel. As a result of this 'cheating' rather than solving, however, they can't easily go beyond their borders.
However, the earthlings have managed to spread out and the newly minted intergalactic race has, over a couple thousand years, managed to inhabit a good 500-2000 nearby planets. There are no aliens to worry about, and mankind has space all to themselves.
But all cannot be so peaceful in the little corner in which mankind has settled; and suddenly the planets upon which our nomadic earthlings have settled start to feel a disturbing little pull. The source of this pull begins on the outer planets near one of the edges, but slowly moves through human territory, enveloping and destroying anything its reach extends to.
Originally, I cited the source of this pulling sensation as the 'Big Crunch' - which has, admittedly, been given the scientific toss. It also doesn't give quite the creeping sensation since it would likely affect all planets simultaneously, rather than starting with the outer ones first. https://en.wikipedia.org/wiki/Big_Crunch
Lately, I've moved to the idea of a supermassive black hole. However, even doubling the number of semi-inhabitable planets within a reasonable range, this black hole would have to be ubermassive. I've done some searching, and found black holes at the center of solar systems/galaxies/you name it. But I can't find the gravitational reach of such a ubermassive black hole. So I can't check this for plausibility. I'm left with the consolation that it doesn't have to suck in planets, just pull them off their orbit (a planet without a sun isn't very good at sustaining life), but I don't know if this is within the realm of physics or will get me laughed out of the publishing industry. https://en.wikipedia.org/wiki/Supermassive_black_hole
So, the question then, is could an ubermassive black hole exist (through natural means or through unnatrual means - it doesn't have to be natural, just self sustaining). And how big would such a one have to be in order to eat/destabilize a galaxy? If a black hole won't work, is there any other source of gravity that could believably cause this much damage while still traveling through space?
Sorry for the Wikipedia article links, they were, surprisingly, the best articles I could find. Most of the others were too advanced or meant for children.
Source for currently known inhabitable planets and their distance from Earth: https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets
Also, this probably goes without saying, but scientific is best. That said, I'll make due with plausible. I just don't want to get laughed out of civilized society.