How could we, in this universe, ever “see” or “detect” another universe, let alone many others? Unless we can so detect one , then this whole thing will always be a theory. Am I not correct?
Now you're really getting into sci-fi territory.
1. A number of the theories used to explain the apparent conflict between relativity and quantum mechanics depend on the "other universe" or "hidden dimensions" theories. It is possible to make predictions based on these theories whereby the not observable things would affect things we can observe. Experiments could potentially prove or disprove these predictions.
2. Some have proposed that at very high speeds or gravitational levels (black holes) it might be possible to "turn sideways" and pass into another universe or at least be able to observe it or its effects.
I have no idea whether any of this is possible. I'm just leary of using the terms "always" and "never" when talking about possible future science.