In a Universe where space is expanding, how do separate galaxies manage to collide? (Strange Universe Series, 2011)

Hubble just took there spectacular images of two distant galaxies interacting (read colliding). Our own Milky Way Galaxy likely faces this same fate. It appears to be on course for a collision with the Andromeda Galaxy in a mere 3 to 5 billion years.

Clearly, if all space were expanding at an accelerating rate, galactic collisions would not be possible. In fact, if all space were expanding, we’d be zooming ever further from our own Sun, and soon become a cold, barren ice ball. So space within a local cluster doesn’t expand. Why? And how large an area can be bound as a cluster exempt from spatial expansion?
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Here are the previous Strange Universe Series—2011 questions:

3—If the universe is infinite, how big is what it is expanding into?
2—How can we be certain the Uncertainty Principle is certain?
1—How do you envision space in more than 3 dimensions, then rotate it to see what happens?

The entire 2010 Series of 20 questions can be found from here.