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Update 12/11/2016: Some atheists in response to this have resorted to appealing to the laws of quantum mechanics and general relativity to attempt to justify their claims about more than one universe existing. There are two problems with that: A, the laws of quantum mechanics govern how subatomic particles interact with each other. Subatomic particles cannot interact with each other if they don't exist, and before the universe existed, they didn't exist; if those subatomic particles don't exist outside space-time — which they don't — then quantum mechanics don't exist outside space-time either. B, the general theory of relativity depends on the existence of gravity, which in turn depends on the existence of mass and density, both of which also depend on the existence of matter — again, matter cannot exist outside the universe. Neither can time and neither can space. Without space, time, or matter existing, quantum mechanics and general relativity cannot exist either; therefore, trying to bring them up and claim that they're evidence of multiple universes existing is circular reasoning.
Also, smashing subatomic particles together in an accelerator like CERN's Large Hadron Collider destroys matter. It doesn't create matter, and it certainly doesn't create space or time. It emulates conditions in the first seconds of this universe's existence, sure, but it does so on an *extremely* small scale, and depends on space, time, and matter already existing in order to do so; therefore, appealing to these experiments and the data associated with them is also circular reasoning.
Original post continues below.
One thing that atheists seem to be very good at when arguing with intelligent Christians like Sean McDowell, Greg Koukl, J. Warner Wallace, and myself is finding ways to circumvent the cosmological argument. If space, time, and matter all had beginnings simultaneously, according to this argument, then the "uncaused first cause" must transcend space, transcend time, and not be made of matter — all three of which are attributes of the God of the Bible. Atheists often just avoid this argument, how exactly? By positing theories like the "multiverse" that on their own merits are even more impossible to prove absolutely — not just impossible to prove empirically but also impossible to prove forensically and archaeologically — than Christianity.
Right off the bat, there's a problem with calling the multiverse a "theory". A theory must be provable by definition. Is the multiverse provable? If it were, then it would be possible to travel physically from one universe to another and back and live to tell about it. That alone is physically impossible proof, why? Because even at the speed of light, our own universe takes *billions* of years to cross. How many years, therefore, would it take to travel from one universe to another if multiple universes exist? Trillions? The astronauts would be fossils by the time they got there — if it was even possible to leave this universe without the spacecraft self-destructing.
Self-destructing? How can that be, you may ask? Because all the constants within the confines of this universe are just perfect for matter to exist. The instant you step outside the universe and enter something else, you enter a place where matter cannot physically exist! What does a spacecraft, at that point, do? There's no forces outside space-time that can sustain matter, so immediately that presents an enigma if talked about from a purely materialistic standpoint. Put short, even if we could travel outside this universe, we might not make it to another universe (if such a thing exists at all) without first running into a complete space-time dead zone capable of destroying all matter at the subatomic level.
Is the multiverse theory still possible? Of course — it's a special case of the appeal to probability fallacy. Is it reasonable? Based on the Christian definition, yes, but based on the atheistic definition of reason, absolutely not. When people posit theories like the multiverse, what they're essentially doing is copping out. They're trying to replace God with some impersonal entity that not only is it physically impossible to obtain empirical support for — just as with the God of the Bible — but it's also physically impossible to forensically support, and, to add insult to injury, physically impossible to archaeologically support. If you have not only no empirical evidence but also no manuscripts or artifacts supporting a theory, then what you have is a theory that takes more faith to believe than Christianity — hence the title of Frank Turek's book, "I Don't Have Enough Faith to Be An Atheist".