Maybe. We don't have the ability yet to fully understand our own universe, so a multiverse is certainly possible.

It's real..though unknowable..

I find the ideas -- there are many variations on that theme, you know -- fascinating. It's not terribly testable with the knowledge and technology we have today. But I like this push of imagination onto the things we are learning.

Which multiverse theory? Bubble universes, higher dimensional universe that exist simultaneously with ours, or wormhole based parallel universes.
It's kind of hard to grasp a bubble universe concept. Parallel universes that spring up anytime a scenario presents the possibility of multiple outcomes would lead to a profusion of universes the vast majority of which wouldn't differ much from the universe they diverged from.

Higher dimensional space with multiple universes existing simultaneously possibly within the same space is an intriguing possibility. This p-brane concept would explain some fundamental properties of reality.

The wormhole universes would be damned cool. Without a whole lot of data to support this possibility, it seems unlikely that we will ever gain access to parallel universes.

Another possibility is that higher dimensions are wrapped up in quantum particles and are to small to permit entry by anything but a graviton would be a damned big disappointment. There would be infinite possibilities for other dimensions to exist all around us, but no possibility of ever exploring that space.

In likelihood parallel universe would exist forever beyond our reach.