There aren&#x27;t any known arrangements of neutrons that are stable without protons. Solo neutrons don&#x27;t exist for longer than 10-20 minutes before decaying into protons and I doubt these tetraneutrons exist for longer than a few seconds unless they&#x27;re moving at relativistic speeds (and even then, for not much longer). Unless there is something drastically different going on in the interstellar medium, neutrons as dark matter is extremely unlikely.