Here in our Universe, almost every combination of fundamental particles you can imagine will lead to an unstable state. Anything containing a strange, charm, bottom, or top quark will decay in extremely short order, as will anything containing a muon or tau lepton, as well as their antiparticles. In fact, the only quark-containing particles that are stable at all are the proton, and other atomic nuclei (protons and neutrons combined) that contain at least one proton. But if a proton and neutron, when they’re bound together into a deuteron, are more stable together than when they’re free, then why can’t two (or more) neutrons bind together to make a stable, bound state of neutronium?
It’s an idea that dates all the way back to 1926: six full years before the discovery of the neutron. It’s also the question of Scott McGregor, who wants to know: