Earlier quoted context omitted.
It's a metaphysical assumption, meaning beyond the realm of experimental science. Science has inherent limitations, and cannot be used to explain everything. This is a good starting point: https://www.youtube.com/watch?v=If3cNUixEBM
How did I know before I clicked that link that it was going to be apologetics. Faith is not a shortcut to knowledge. "Hey, we can't yet find the answer to big questions through rational means, so let's try irrational means!"
Islam has proof and evidence (e.g. http://provingislam.com, https://www.youtube.com/watch?v=nTsEZXx8kRg, https://www.reddit.com/r/islam/comments/hxn276/here_are_some..., and much more). Don't present a false dichotomy between science and faith where there isn't any for us in Islam.