If the universe is constantly expanding and everything is moving away from everything else then why are galaxies

Question: If the universe is constantly expanding and everything is moving away from everything else then why are galaxies colliding?
- Keywords:
  space
Asked by lizzie3006 to Ben, Jony, Katharine, Mark, Peter on 20 Nov 2011.
- Peter Williams answered on 18 Nov 2011:
  
  when you average over the motions of the galaxies a certain distance away you find that the average is increasing. but individual galaxies are still able to collide
- Ben Still answered on 18 Nov 2011:
  
  They collide as they are attracted to each other by gravity. Gravity clumps matter – stars, planets etc
- Mark Basham answered on 20 Nov 2011:
  
  Here is a movie of this process happening, Its quite cool as we use these simulations and compare them with observations to try to work out whats happening.
  
  Also there is a quite long-winded page about it here
  
  http://coldcreation.blogspot.com/2011/01/large-scale-structures-superclusters.html
  
  Figure FCLSF, is the interesting bit though, search for it with Ctrl-F

Question: If the universe is constantly expanding and everything is moving away from everything else then why are galaxies colliding?