Knowing is different than proving. Knowing means,
- to perceive or understand as fact or truth; to apprehend clearly and with certainty: I know the situation fully.
The question is not how do you prove something. It is "how do you know that you know?"
Your answer was the best I heard. Most people spontaneously combust when they attempt to figure it out.
I understand where you're coming. We cannot say that we KNOW every complex detail of nanoscience because science and our ability to observe and understand the world in greater detail are ever advancing.
To know something is to first observe the phenomenon and then analyse our observations into a rational understanding. Once we have this rational understanding, if we cannot objectively produce any evidence to the contrary, we call it a theory, which means a fact TO THE GREATEST EXTENT OF OUR CURRENT UNDERSTANDING.
Oftentimes people think the word "theory" means unsubstantiated, but using the word theory instead of fact is science's polite way of saying, "but feel free to prove me wrong, we may find evidence to the contrary as science advances."
To summarise, we produce facts by making observations that we analyse. A fact is a theory which can (and has) been verified many times.