Modern physics has devised two theories to account for most of the physical phenomenon in the observable universe around us: quantum theory and general theory of relativity. These two theories have given commendable results and are not known to be wrong in any of their predictions in the realm in which they are applicable. Thus both the theories are accepted as 'correct' theories. However these two theories have some fundamental differences and acceptance of both the theories despite these fundamental differences have even led many of us to believe that 'universe' is 'divided' into two realms, one microscopic wherein quantum theory is applicable and second macroscopic wherein general theory of relativity reigns supreme.
In addition to this artificial division based on scale, there is an issue of ontology of Quantum Physics. Though quantum theory has been phenomenally successful in numerical predictions, it has resisted ontological explanation. So much so, ontology has been shoved under the carpet and very 'reasonableness of searching for ontological explanations in science' has been challenged. Explanation like wave collapse or multiple parallel universes branching out continuously or multidimensional universe or even the consciousness of observer playing an intimate part in physical events blurring the role of subjectivity have been uncomfortable to say the least, at least to the beginner and slowly a 'sense of quantum theory' is developed in the student of physics which is euphemism for getting overwhelmed with numerical successes with pure epistemological considerations getting rewarded.
Though there has been a Bhor’s Correspondence Principle in play taking quantum physical effects towards classical physics effects as the scales of time and space or quantity of individual particles and/ or events move upwards, it is also getting realized that difference in ‘principles’ like wave-particle duality, multiple path motion of an individual quanta etc. may not get ironed out so smoothly by upping the scales or increasing the quantities of particles/ events participating in any phenomenon. Also some of enigmatic quantum phenomena like entanglement have been shown to exist even at macroscopic scales. In fact, at the roots of attempt to explain quantum ontology by proposing multiple dimensions of Universe or by envisioning multiple universes themselves, is the recognition of fundamental differences in two theories not reconcilable in an ordinary way.