We recently observed a new kind of collective excitation called a “demon”. This “particle” was predicted by David Pines in 1956, but it was never observed until we saw it in Sr2RuO4 in our momentum-resolved EELS experiments. Demons are interesting because they are massless and neutral, and do not couple to light, but can have strong influence over the low-energy properties of multiband metals. For example, Ihm, Cohen, and Tuan showed in a theoretical paper in 1981 that demons could mediate superconductivity. While not every multiband metal necessarily contains a demon, they are probably not rare, and represent a new phenomenon in condensed matter physics. For more details, see our paper in Nature.