Unknowns in Physics

Occasionally my A level physics students ask me questions which I cannot answer.  For example:

• What was there before the big-bang?  
• Why do we have this set of elementary particles as opposed to some other set?
• Why do masses attract? (in other words why does gravity exist?)
• Why do we have the four forces (interactions) of nature? how did they come about?

Much of physics is about describing the behaviour of the world (and the universe) at its most fundamental level. But these descriptions often don't tell us why it is that nature behaves in that way. 

Here's an example; Newton's law of gravitation was a huge achievement for science and mankind.  The law is often stated in the form that the force of attraction between two masses is proportional to the product of their masses and inversely proportional to the square of their separation.  This relationship has been shown to be correct to an extremely high degree of accuracy. 

But scientists (and especially physicists) like to have a mechanism for how this force of gravity comes about, from which the above relationship would then follow as a consequence.  However we often don't have (or understand) the mechanisms behind various phenomena of nature.

To quote the Nobel prize-winning physicist Richard Feynman on the subject of quantum mechanics:

"It is my task to convince you not to turn away because you don't understand it.  You see, my physics students don't understand it either. That's because I don't understand it.  Nobody does."

I would just add that I do fully understand A level physics!