how did the first theory about axions come about?

Question: how did the first theory about axions come about?
- Keywords:
  inspiration,
  physics,
  research
Asked by daviesl to Francesca on 12 Mar 2014.
- Francesca Day answered on 12 Mar 2014:
  
  Axions were first thought of to solve a problem called the “Strong CP problem” about how different particles behave. Then people realised that string theory also predicts that axions exist. Dark matter might even be made of axions!

Question: how did the first theory about axions come about?