It doesn't matter. Though I'm sure that if it is true there's probably a version of me who thinks otherwise. At the end of the day though, we have no way of detecting, observing, or interacting with any other universe and no evidence we possess actually suggests the existence of them as anything more than a thought experiment or fictional concept. As such, whether or not they actually exist doesn't actually matter as our universe is apparently independent from any other universe that might exist, incapable of affecting or being affected by others. So for all intents and purposes our universe - to the best of our knowledge - acts as if it is the only one in existence.