Strictly speaking, it is not demonstrable that there is a domain of demonstration. We have to arrive at an agreement that there is a domain, or possibility of demonstration, before we can accept the value of any demonstration. This is similar to a very famous philosophical point made long ago by David Hume, that our belief in the value of inductive reasoning rests upon an unproven assumption that the future will resemble the past in all appropriate ways. I'm not sure if you have run into that argument but it is very interesting. Consider a normal scientific investigation. We test the boiling point of water and find it to be 100o Celsius at normal air pressure. We test it many times in many different laboratories and lots of people confirm that this is the case. Will it continue to be the case tomorrow? We assume that it will. But maybe water will be different tomorrow. We assume that we live in a consistent, predictable universe, that does not alter the way it functions, and of course, that is what we have always seen. But that does not prove that the universe cannot arbitrarily change, just because so far it hasn't done so. I like to think that it won't, but we really cannot prove that it won't.
My sister (who is a lovely person, but a bit strange) believes that the universe as we perceive it is a dream, from which we will some day waken. This is a perfectly consistent view of reality. There is absolutely nothing that she could ever experience which would prove to her that her life is not actually a dream, because anything she experiences could be part of a dream. Of course, we do not normally expect dreams to have experiences beyond a certain level of intensity, such as severe pain. But that could just be our own false preconception. Even though certain things would cause us to wake up from our normal dreams while we are sleeping, it is perfectly logical to imagine that when we wake up from that dream, we merely enter into a different dream, which is more vivid and can include more intense experiences. There is no way to disprove this although I believe that it could be criticized as an unnecessary hypothesis (which is also the most applicable argument for believing that the universe was not created by God).
When I said "skepticism is not only about what is clearly demonstrable" I was trying to convey the idea that skepticism can arise even in cases when no clear demonstration has taken place. Of course, we would not be skeptical about things that are clearly demonstrated. We would be skeptical about the rejection of things that have been clearly demonstrated. But we can also be skeptical about other things. I have all kinds of opinions, many of which pertain to speculative matters that have not been clearly demonstrated one way or the other. Will there be a human colony on Mars this century? Nobody can prove either that there will or that there won't, but I personally am skeptical. By the time the century ends, in another 85 years (by which time I will be dead) there will be a definitive answer. And there might be one sooner. Or there might not.