To my belief, Each galaxy is being held together by certain forces, the main ones being gravity from a super massive black hole in the center of the galaxy, and also dark matter within the galaxy itself. What i think they are saying by "technically shouldn't exist" is that since black holes are a point in space, its hard to comprehend how something so big is being held together by a point. This mostly points to the idea that dark matter is holding it together. This is kinda hard to comprehend because of the tendency for a galaxy to continuously spin creating a greater force pushing outwards making the galaxy more likely to break apart. If you factor in one point holding it together, the force pushing out by speed, and the actual size of this galaxy, it mostly doesn't make much sense as to why its still together. Like i said, the only real answer i can think of why it is still together would have to be dark matter and also dark energy pulling the galaxy together. I haven't learned to much on those two subjects so that's my best guess. Its hard to think that something even as big as our galaxy is being held together by only a few forces, and to think of a galaxy that is 8x populated being held together is even more crazy.