I partially agree with you. We aren't at some type of zenith of human knowledge, and people who think they know all the answers are idiots, or dishonest. But, as far as one could know anything, there are some things we do know.
A THEORY in science isn't just some kind of guess. Theories are ideas supported by so much evidence and so many facts that they are practically considered true. Of course we don't have absolute knowledge, but as far as practical knowledge, anything that's considered a theory is a known fact about the universe. The theory of gravity isn't less true because it's considered a theory. It's like saying germs don't exist because germ theory is JUST a theory. It's an easy mistake though, because of the way we use the term theory in everyday speech. It doesn't have the same meaning when used in a scientific context. Plus, throw in the piss poor science education in schools, and it's easy to see why someone would say something like that.
Men thought the Earth was flat because it seems intuitive. Using your senses today you can't tell you're walking along the edge of a spherical object. That's why theists kill me when they say certain scientific concepts don't make sense logically (only to conclude that God is the only rational explanation for the universe/diversity of life/etc.). What "makes sense" or "seems impossible/improbable" is not necessarily the best pathway to truth. Sometimes what's true about our universe, like a round Earth, is counter-intuitive.