&gt; no multiverse theory has yet been put forth that is experimentally testable<p>Just to be clear here; that&#x27;s because there <i>is</i> no theory for a multiverse. Not yet, anyways. Nobody has put one forth yet. When you hear &quot;multiverse&quot; come out of physicist&#x27;s mouth, it&#x27;s because it&#x27;s a concept indirectly related to other theories. The current popular theory which involves a multiverse is string theory. When string theorists do the math, there is some evidence that a multiverse is possible.<p>However, that doesn&#x27;t mean much. Even if string theory was correct and little strings are really the fundamental component of everything in the universe, the multiverse part of string theory could still be wrong. The theory isn&#x27;t reliant on it, it just doesn&#x27;t forbid it.<p>I also wouldn&#x27;t say that it&#x27;s entirely untestable. There are a couple things that could be indicative of a multiverse that some physicists have looked for: <a href="http:&#x2F;&#x2F;phys.org&#x2F;news&#x2F;2010-12-scientists-evidence-universes.html" rel="nofollow">http:&#x2F;&#x2F;phys.org&#x2F;news&#x2F;2010-12-scientists-evidence-universes.h...</a><p>The source isn&#x27;t the greatest, but it shows that we can look at the CMB for indirect evidence. With higher resolution scanning years in the future, such a theory may be testable. I only mention this because the way your comment reads, it sounds like you&#x27;re saying a multiverse would be inherently untestable.