>But what is really causing this stuff? What is it made of?
The electromagnetic field is one of the fundamental forces of nature, and it is mediated by photons. It is known in detail and we can predict its behavior to uncanny accuracy by means of quantum electrodynamics.
However, I am afraid no answer will be good enough for you, that you want to understand it as good as you "understand" other concepts such as "rock" or "cat". The EM field is inherently different, (for example), in that it acts at a distance. And what does it means that you understand something? Do you understand what a cat is?
Your last paragraph has a few sentences that I disagree with, but particularly : >In the end there is nothing traveling through nothingness and all is nothing and no one knows anything.
Where my version would be: In the end there is matter traveling through space and all is nature and we don't know the whole story yet, but we can predict many things with unbelievable accuracy and make lots of useful things.