Quantum theory is the theoretical basis of modern physics that explains the nature and behaviour of matter and energy on the atomic and subatomic level.
One of the biggest goals in physics is to answer the seemingly simple question 'What is the universe made of?'. For the last few centuries, we have found a better and better answer to this question but there is still lots of work to be done. For example, if a human is made of molecules and those molecules are made of atoms then what are atoms made of.
Currently there are 4 fundamental forces that we know of in the universe. Gravity, electromagnetism, the weak nuclear force and the strong nuclear force. These forces combine everything in the universe and without one of these forces all existing matter would rapidly change.