There are basically two routes to so called 'scientific' multi-universe or alternate reality theories, from the cosmology and from quantum mechanics. According to general relativity Spacetime is shaped by matter. The universe that we are part of is a particular shape. Some models are closed shapes, some are open. In either case the universe is not infinite because spacetime cannot extend very far beyond the matter that shapes and creates it. It is difficult to imagine the universe as a closed four dimensional shape without picturing it as 'floating' in something, perhaps another infinite space. However this image is not justified by the science and implies further dimensions for the four dimensional universe to be in. If someone wished to speculate about this in a non-scientific way they could imagine an infinite number of 'island universes' dotted about in this multi-dimensional space much as galaxies are spread out in the four dimensional space that we know about. However these universes could have no contact with each other and could not exchange information (I am aware that there are 'wormhole speculations about transference of information but these are ot based on testable science and presume that alternative universes exist to create wormholes to). From a scientific point of view they could not be detectable and hence effectively do not exist. In many scientific theories there are often terms of equations that are neglected for this very reason. Non-interaction is effectively non-existence. It is in this unexplorable realm that believers place their gods and it is no more justifiable for scientists.    The many universe quantum theories are an attempt to resolve problems of superposition of states. When these states decohere there are problems for classically deterministic systems. The supposed solution offered by multi-universe theories is that all possible states of a quantum system lead to real observable solutions in alternate universes. This implies that effectively an infinite number of mature alternative universes come into being every moment as a result of each alternative state cross-referencing with every other alternative state. There are many more problems with this than the obvious one of where does all this matter and energy come from and in what ‘space’ does it exist. A damning consequence for the speculation is that every possible alternative universe will exist. This means that even if there were a experimentally testable consequence of this hypothesis there are universes where scientific experimentation always fails and hence 'proves' that the hypothesis itself is incorrect. As a solution to anything this hypothesis is not only untestable but also must refute itself. Whatever this hypothesis is it is not a scientific one and has no meaning in terms of explaining experience. Given that there are at least ten alternative hypothesise to resolve the problem of interaction between quantum world and the classical world that do not require the creation of infinite universes at each moment in time (i.e. everything happens because everything is possible) it is unnecessary to leap to an unscientific metaphysical one that creates more problems than it resolves (personally I favour a variation of einselection).