The brain-in-a-vat -concept points to the question, how do you know that you are not really living in a awfully sophisticated virtual reality? You have no direct access to the universe itself, you have to experience it through your sense and those can be fooled. This is an ancient old question debated in Philosophy in many forms since at least Descartes. Matrix has made this idea ‘pop’ in recent years.
If you are a brain-in-a-vat how could I find something like that out? Of course, without direct access to the outside world, you can’t know anything about it, but there are ways to suggest that the reality you think is real is not. Of course these ways come in different degrees of certainty and you can never be absolutely sure, but absolute certainty is rarely an requirement outside philosophers, others are usually content with bloody damn sure.