Untestable Hypothesis and Falsifiability - The Scientific Method has some five key steps to it...
1. Observation/Question
2. Hypothesis
3. Prediction
4. Experiment
5. Conclusion

One of the toughest parts of the Scientific Method is simply determining whether it's possible to design an experiment to test your hypothesis. If it IS possible to test it, and there are clear conditions for what counts as refuting your hypothesis, the hypothesis is called falsifiable, and this is a good thing in science. "Falsifiable" means the same thing as "testable," it doesn't mean "proven false." Yeah, it's confusing. So some examples may help.

Hypothesis: There are NO black swans.
Test: Look for a black swan.
Falsifiable? Yes
Potential Falsification: Finding a black swan.
Truth Status: False (there are black swans).

Hypothesis: There are pink elephants.
Test: Look for a pink elephant.
Falsifiable? No
Potential Falsification: None. If you looked around the whole world, maybe it was hiding in Japan while you were in New Zealand. If you saw the whole world simultaneously, maybe it's on Mars. Or another solar system. It's impossible to actually carry out the test.

Hypothesis: The First Kirkin of the tartan was done in the USA.
Test: Look for evidence that there was never one done before
Falsifiable? No
Potential Falsification: None. If you look at every possible source there is still a chance it happened but was not recorded

Truth Status: So far it appears false, but we're not sure. The statment "there is no such thing as a pink elephant" is a good example of something that a non-scientist would call a fact but scientists would say is unproven.