Three quarks for Muster Mark!
I don't recall who said it, but to paraphrase, the most amazing fact about the universe is that we can comprehend it and our place therein. The universe we observe has rules that can be described by human invented mathematics. Working scientist don't give much thought to "what is mass". It's a word that describes phenomena that they can predict and measure. Only philosophy majors, usually after a few drinks, give a thought about what it is.