Since first hearing about dark energy in the popular press I’ve always had the impression that we can’t detect it because it’s some exotic kind of energy that doesn’t interact with matter. Is it true that it doesn’t interact with matter or is that just a popular misinterpretation? I mean I know we can’t detect it, but I think we also know that expansion is halted in the presence of matter within galaxies. So isn’t that an interaction of sorts between matter and DE; the fact that expansion is stopped. Is it more the case that DE exerts a uniform force everywhere and so gives no variations against which to test?