I read a blog called Slate Star Codex, and the most recent post is about “Is a multiple universe theory scientific?” https://slatestarcodex.com/2019/11/06/building-intuitions-on-non-empirical-arguments-in-science/
On the one hand, there’s no evidence. This leads people to say it’s not scientific. On the other hand, a single universe theory doesn’t explain everything and a multi-verse theory explains more. In support of the second position, Slate Star Codex posted: https://slatestarcodex.com/2019/11/18/more-intuition-building-on-non-empirical-science-three-stories/
Which has two examples of non-evidence based theories that we currently believe. 1. We believe that mixing an acid and a base creates just a salt and water, and not salt, water, and a supernova somewhere in space that’s untestable. Even though it’s not testable, we choose to believe the simpler idea. 2. We believe that fossils came from dinosaurs and not the devil planting them even though the devil theory is simpler. The point is that it’s not just about a simpler theory.
This is missing the big picture. The whole point of science is that it’s predictive. The difference between science and art, is that science predicts the future. It’s an explicit if-then statement about nature. If you water your plants, then they will grow (holding all other variables constant, of course). If there is a supernova everytime someone mixes an acid and a base, then we should see a spike in cosmic radiation. If the devil planted bones, then we should find the devil’s shovel, or mounds of disturbed dirt around the bones, or all the bones should seem reasonably the same age regardless of where they are found.
The multi-verse theory isn’t scientific, not because it lacks evidence, but because it’s not predictive. If someone says we live in a simulation, we should see evidence of that. I should be able to predict that we’d see glitches in the Matrix. That kind of non-evidence based speculation isn’t science, it’s just a great book.