I can count on one hand the number of times that I've heard someone worth listening to say "well, science doesn't know everything" in response to anything I've said or heard someone else say.

If the conversation goes:
A: I believe X.
B: X is contradicted by (citation to some study).
A: Well, science doesn't know everything.

then there is essentially no chance A has anything interesting to say about empirical topics - at least those unrelated to that person's job.