The universe is expanding, but that does not mean that internal interactions between galaxies do not exist! In places where the force of gravity between galaxies overpowers the general force of expansion (since the expansion is accelerating you can imagine that it would generate a force on the galaxies), galaxies will drift together.

Imagine a huge firecracker filled with magnets. The firecracker explodes, and sends magnets flying everywhere. There is some randomization in the paths of each magnet, so some magnets are farther away from each other than others. If some of the magnets are close enough, they will stick together, and then continue on the path that the explosion set them on. This is basically the case with  the universe, except we are talking about gravity rather than magnetism, and we are talking about acellerated expansion rather than an explosion.

Ok, I'm shooting from the hip here, so take everything I say with a grain of salt.  However, this is the way I have roughly come to understand it.

Not every galaxy is speeding away from every other galaxy.  There are clumps of galaxies and the clumps are speeding away from each other.

Our "clump" of Galaxies is called the Local Group.

Within the Local Group we have two major Galaxies and roughly 30 minor galaxies.

The two major Galaxies is ours(Milky Way) and I believe Andromeda.  Both of these galaxies will eventually crash into each other and become one super huge Galaxy.  And I guess eventually all the minor galaxies will eventually get pulled in too.  eventually everything will get all sucked down into one supermassive blackhole.

Anyways, that's a layman's way of saying it.  Cause I'm definetly a layman with this kind of thing.

http://en.wikipedia.org/wiki/Local_Group

Actually it is not expanding as if from a an explosion, it is expanding as in "just sort of getting bigger". Imagine the universe is on the surface of a balloon which is expanding. The galaxies are like magnetic dust. They wander around under their local attraction. On average they all get further apart, but some local collisions occur. The expansion thing is rather smooth and almost imperceptible on the mere galactic scale. The average distance between galaxies is about 1/10,000 of the observable universe so their interactions with each other are indeed very "local". As for the galactic central black holes, they are as moveable as anything else. They are just very heavy so nothing much else moves them, although the weight of the whole galaxy can hold them. When galaxies collide and merge they should end up with two black holes but this area of observation is quite new (it has been about 10 years since the observations of black holes in every galaxy) and so what happens to them in collision is under active discussion. You might think they would collide but actually their chances of doing so are quite small, they could tango in orbits for billions of years unless there are some effects that provide drag. And just to add to the fun, it is also becoming clear that almost all medium to large galaxies are formed by mergers of many small galaxies, several if not all of which had black hole cores of some size. Our own galaxy is now known to have swallowed quite a few neighbors. Where those cores went, no-one yet knows.

No, the universe is accelerating in expansion because of dark energy, which acts to counter gravity, hence the effect of mass. The divergent motion of material bodies is the result of energy that has been within the system since the Big Bang. Under a more massive system, gravity will move to counteract this and so, eventually, the system will stop expanding and start acceleration in recession. This is not what is happening because there is not enough mass.

This is a horrible misconception. There is nothing outside the physical universe, which we shall define as contained within the boundaries of our spatiotemporal manifold, that "feeds" it.