There are only a very few people from the entire history of our species who have run particle collider experiments and verified first hand what's inside an atom. What they agree on is truth for everyone because almost nobody has the means to test it themselves. And then of course this truth is modified and updated as we find more data. Then old conclusions are rejected and the entire baseline of truth changes.
We can be sure of things to however many decimal places as you'd like, but reality itself is fundamentally built on probabilities and error bars. What we think we know is built on probabilities on probabilities.