The neutrino in question, known as a sterile neutrino, was supposed to only interact with gravity and have zero interactions with matter. Physics can (at least in theory) extend far beyond the boundaries of the Standard Model. A hypothetical particle thought to never interact with matter is one of them. Sterile neutrinos are even weirder than standard neutrinos because while neutrinos barely interact with matter as it is, sterile neutrinos were thought to not to have any interactions with matter at all. Before this experiment, the existence of sterile neutrinos was the explanation often accepted for the neutrinos behaving strangely.