Where to Stop the Buck?

Short note.  In Philosophy class, we were going over Descartes' Mind/Body dualism and the question posed by Princess something-or-other to Descartes: :"By what means can a pure thinking substance (mind) cause a material substance (body) to move?  After some banter back and forth, and progression on to Malebranche's occasionalism, I noticed that there's always the next question.  What I mean is, you can continue asking "why" and "how" of every explanation given for any state of affairs ad infinitum.  As some atheists love to do in the following manner:
Q. "How did the Universe come to be"?
A. "God made it"
Q. "Who made God?"

You see what I mean.  But here's the thing: The Scientific endeavor that is now in progress won't take "just because" for an answer on anything.  There must be an explanation for everything, every process, every motion, every force, EVERYTHING.  Yet Science is in search of a "Theory of Everything" (T.O.E.).  But if Science remains consistent, it can never have a TOE, because the moment it decides that everything reduces to strings or whatever, then there is still the inevitable question "what causes those thingamajigs to exist and act that way?" and the search must go on infinitely.

So it seems to me, as useful as this habit of questioning has proved to be, there must be at least one place, possibly many, where the buck stops, where the questioning process itself is meaningless, where we have something, someONE, of Whom when asked "Why does He/It exist and function in such a way" the only answer possible will not be "because of this or that internal mechanism/thingie" but must simply be "just because" - NO FURTHER EXPLANATION POSSIBLE.

Something like when you have a guy who sees a flaming bush that tells him to take his shoes off and go tell the Egyptian dictator to let the Hebrews go, and the guy asks "Who should I say sent me", gets the answer:

I AM THAT I AM.