RE: You just can\'t...
Whoa! Science believes it understands the nature of the Universe? I have to argue that. Maybe a very small minority of people think that all the questions about the fundamental properties of Nature are solved, but in reality, science is a constantly changing body of knowledge. I don't think that you could find a single physicist, for example, that thinks that Quantum Mechanics is the be-all and end-all of physical theory. Every theory is created to explain as much as possible at the time, and make predictions based on current knowledge. At any time solid evidence of something contradicting theory could completely mess up everything in the current theory. No-one serious ever thinks that we actually understand anything on a fundamental level.
On a different topic, the problem with saying "Time doesn't exist" or "it's all in our heads" is that we're so completely linked to our heads. I mean, if time doesn't exist, but we all experience this non-existant thing in pretty much the same way, and can predict and observe its effects pretty well, then doesn't it, in effect, exist? But then the discussion becomes philosophy.