What causes the strong nuclear force that keeps protons and neutrons together in a nucleus?

Question: What causes the strong nuclear force that keeps protons and neutrons together in a nucleus?
- Keywords:
  force,
  nuclear,
  particle
Asked by Zealousy to Rob, Chris on 21 Jun 2015.
- Rob Temperton answered on 21 Jun 2015:
  
  Blimey… This is a long way out of my specialism!
  
  I don’t really know much about ur myself, but it is thought that the gluon particle mediates the strong force. There is an area of physics called QCD (quantum chromodynamics) that describes the interactions between quarks (the particles which make up protons and neutrons) and gluons.
  
  Rob

Question: What causes the strong nuclear force that keeps protons and neutrons together in a nucleus?