Today what is possible thanks to advancement of technology was only real in sci-fi texts times before. (*)
You could replace "times before with "in the past", "in past times", or "just a few years ago".
You could also move things around and say something like
What is possible today thanks to advancements of technology was previously only real in science fiction.
I also made a couple of other changes:
we say "the advancement of technology" or "advancements in technology", but not "advancement of technology" without the article.
"sci-fi" is considered somewhat derogatory (at least by sci-fi fans).
"texts" was an unnecessary restriction---didn't sci-fi films also show some of those impossible things?
"Today what is possible..." sounds clumsy, although I can't say for sure why. Probably it's a preference to have a single phrase before the verb.