if the galaxies are flying apart isn’t that because of the big bang? if not why?

Question: if the galaxies are flying apart isn't that because of the big bang? if not why?
- Keywords:
  big bang,
  galaxy,
  space
Asked by sammy1 to Clare on 14 Mar 2012.
- Clare Burrage answered on 14 Mar 2012:
  
  Yes galaxies are flying apart because of the big bang explosion at the beginning of our universe. But what we expect is that this should star to slow down.
  However what we see is that not only is the expansion of the universe not slowing dow, it’s speeding up! And this is the big mystery, because nothing we understand in physics can explain how this can be happening.

Question: if the galaxies are flying apart isn't that because of the big bang? if not why?