I like Neil Degrasse Tyson’s take on this one. At first we thought there was only one Sun, then we figured out the sun was a star, like so many others. Then we thought there was only one galaxy, then we figured there are much more than that.

Now we think there is only one universe, but it seems nature isn’t very good at creating only one version of something. It’s not unreasonable to think there are many universes. We just can’t prove/observe it yet.