Does parallel universe really exists or just science fiction which ain't possible? Can we ever know?
Multiple and parallel universes are a feature of a number of "theories", including most famously (and probably before string theory), the many worlds interpretation of quantum mechanics.
The attraction of the many worlds or multiverse ideas is that they offer a ready explanation of why we appear to live in a uiverse that has been "fined tuned" for our existence.
The disadvantage of the "theory" is that there does not appear to be any readily available means to falsify it. For that reason many scientists view it as unscientific.
As much as parallel universes go, there's a theory for this known as the Multiverse theory, part of the String theory.
In String theory, the Multiverse is a theory in which our universe is not the only one; many universes exist parallel to each other. These distinct universes within the multiverse theory are called parallel universes.
There are cosmic inflation models that support this as said here in Wikipedia. A quote from there:
It's hard to build models of inflation that don't lead to a multiverse. It's not impossible, so I think there's still certainly research that needs to be done. But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking [the idea of a] multiverse seriously. Alan Guth
Now to address your point if we can ever know if a parallel universe exists exists.
Opinion: Possible but unlikely. I don't think that we are close to achieving this.