Materialists/physicalists seem to think you should only believe something that's provable. This limits belief to the subset of the physical universe provable by the scientific method.
An example of something not provable but worthy of belief: You meet someone you think you will become lifelong friends with. Certainly there is no scientific proof involved; merely intuition. It's OK to believe you might become lifelong friends with them.
"Might" and "definitely will" or even "probably will" are different things. Yes, it's perfectly reasonable to believe that someone you just met
might become a lifelong friend, if for no other reason than you know that lifelong friends at some point meet for the first time.
To determine whether it's
probable or
guaranteed that a person will be your lifelong friend, you need lots more information.
What about scientific speculation; for example, multiple universes? Why do scientists even think about such things if they can't find a way to prove them? It's because they believe they could be true with no evidence they are true.
Again, there's very different criteria involved in determining that something is
possible and determining that something is factually true. I'm no expert on multiple universes, so I'd ask the people who engage in research on it for a living.
The existence of the subjective experience of consciousness deviates from the scope of provable science enough to warrant believing it is outside the physical domain. After all, all other emergent properties (such as the surface tension of water) are still physical, having atoms obeying the laws of physics. Yet there is no quantum field called "consciousness", so it may not be physical in essence.
May not/is not issue again.
Let me ask this: is it justifiable to believe that anything that
could be true, actually is true? If not, why not?
Second question: how do we investigate anything at all about something beyond the physical universe? We only have access to the information provided by our five physical senses.