The multiverse hypothesis is in the philosophical phase of discussion, but it may eventually, slowly, develop into scientific study-we need to find something to measure. A troubling question is whether each universe had its own singularity, or did the singularity of one universe cause all others to come into existence? In considering this hypothesis, we must also wonder if the multiverse is infinite and eternal. Was it always in existence, is there an endless number, and will it last forever? The popular theory of the structure of the multiverse is one of soap suds that form in a bath, each soap bubble represents a universe. This structure theory is based on the principles of randomness, which is perfectly reasonable to assume. It can also be argued that each universe is a sphere of matter that floats in an unknown space, along with the spheres that make up other universes-something like ping pong balls moving around in a lottery machine. These visualizations are interesting to think about, primitive though they may be, but it seems too early in the discussion to try to form structures, we must first find out if it exists at all.
With the choice of possible, probable, likely, and impossible, I'd say the multiverse is likely. So many things are "possible," so it seems too loose of a term for describing the multiverse. Probable and impossible are filled with too much conviction on each side, there is too much still unknown to make a statement of that type. The existence of a multiverse is likely, considering what we have discovered about this universe, it is easy to see that it could be something that has replicated and expanded to a size beyond our comprehension.
An idea of this magnitude goes through the same process as other large and expanding questions. The idea starts with philosophy, proposing ideas, then through a long and laborious process the idea will form into localized theory, then will eventually reach the level of scientific study. This question will not be answered by philosophy, it will be answered by empirical evidence through scientific requirements. To put it another way, we won't "think" of the answer, we'll have to be able to see it, through measurement and formulas most likely. We discover trouble in suspecting that we will not understand the multiverse until we adequately understand this universe, which will take many more years, if all of its secrets will ever be discovered.
The task of the philosopher is to highlight all reasonable possibilities, not necessarily logical possibilities, because quantum mechanics has shed light on how classical logic can be wrong. Particles can and do pop in and out of existence spontaneously, which classical logic tells us cannot happen. Thinking must be adjusted in order to incorporate the demands of quantum mechanics and current cosmological advances. Perhaps the most important job of the philosopher in this case is to produce ideas that will eventually be discarded, whittle the options down by a process of elimination. The philosopher must develop their best theory in the hopes of it being incorrect, it will help discover the answer in a more efficient way than trying to prove the theory by stretching assumptions. It is not only ok to be wrong, it is a necessity.
It has been argued that if the multiverse exists, it will therefore carry the label of infinite, but that does not necessarily need to be the case. What if empty space outside the multiverse exists, wouldn't that dictate that the multiverse has room to grow, and therefore is not infinite? Again, to put any qualitative or quantitive label on the multiverse is premature, to say the least. There are many "what if's" when thinking about the issue, but we must not get lost in questions, losing the real goal, to find out what's true and what isn't true. Considering the vast nature of the question, we must always show great patience, lest we become over eager and follow a path of thought in haste, which could set the study behind hundreds of years. Every step must be carefully examined before we set foot on it to continue up the staircase of discovery.
We are explorers by nature, human history has proven that to be true on many occasions, and we will no doubt have aspirations to travel to other universes should they be discovered. The problem with this curiosity is that another universe may have different laws of physics, in fact, it is reasonable to assume that it would, and our bodies and matter will not be able to survive under such conditions. We may reach the point of awareness of other universes, but will most likely never be able to travel to such places.