The situation of crossing from one theory to another as the scale changes is the situation we're in at the moment, some of the more interesting unanswered questions are in areas where the scales overlap.

Normally quantum effects only happen at the very small scale, and normally gravitation is such a weak force that it is normally only measurable on large scales, where big masses are in play.

Interestingly, there are situations where there are overlaps, one is with the proposed quantum black holes, since the (GR) theory of how black holes behave (which fits the behavior of some observed very heavy objects) don't actually set a lower limit to their mass (once they are somehow created), so it's (GR) theoretically possible to have a black hole with a mass of a few protons. The thing is that the radius of this black hole as defined by GR is so miniscule that the quantum uncertainly of posision of those protons would be enough to move the proton out of the gravity of the black hole which seems to be a contradiction with GR.

I'm actually mainly mentioning this because I don't think it's really possible to defined seperate realms for the two theories and expect no areas of overlap where they will contradict each other, so for an explanaion that involves "everything" something other than both, while contradicting neither in their respective realms, is required.

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‘To those who regard “crime fiction” as some sacred icon which must follow a rigid formula, I will always be the man who writes 18-syllable haiku.’Trying to make sense of computers, The Error Log.