Why does the universe keeps on expanding?

Question: Why does the universe keeps on expanding?
- Keywords:
  big bang,
  expand,
  galaxy,
  solar system,
  space,
  universe
Asked by testvanadiumj11 to Alex, Jools, Lynz, Matt, Rika on 22 Jun 2011. This question was also asked by .
- Julie Greensmith answered on 22 Jun 2011:
  
  One of the theories of this is because of something called entropy – which is the tendency of a system to move towards a more chaotic state.
  
  Entropy is like my bedroom. If I dont put energy into the system (i.e. tidying my room) then stuff starts to spread out and go everywhere. The universe is pretty much the same. If energy is not being put into the system in order to keep molecules together, then they start to drift apart. This mass drifting apart is proposed to be one o the reasons why the universe is expanding.

Question: Why does the universe keeps on expanding?