Answer the following question given this paragraph: 

The existence of quarks was first proposed in the 1960s. Since then, scientists have done experiments to show that quarks really do exist. In fact, they have identified six different types of quarks. However, much remains to be learned about these tiny, fundamental particles of matter. They are very difficult and expensive to study. If you want to learn more about them, including how they are studied, the URL below is a good place to start.


Q: The existence of what tiny, fundamental particles of matter was first proposed in the 1960s?


A:
quarks