Are there multiple universes?

Over the past decades, the idea of other universes has progressed from the realm of fantasy and speculation to the pages of physical journals. But as yet, no-one knows for sure if our Universe is unique. There are many ways in which multiple universes could theoretically exist. They could split off from each other, or exist parallel to each other, or follow each other in time. Some physicists believe that the idea of a multiverse solves the problem of the uncanny fact that our Universe appears to be custom-made for complexity and life. But for other scientists such discussions are purely speculative, as we cannot prove them.