Neutronium is a theoretical substance composed almost entirely of neutrons, which are subatomic particles found in the nucleus of atoms. It is believed to exist in the extreme conditions found in neutron stars, where the gravitational pressure is so intense that electrons and protons combine to form neutrons. This results in a dense material that is incredibly heavy, with a small volume. Due to its unique properties, neutronium is not stable under normal conditions and cannot be found naturally on Earth. Scientists study neutronium to better understand the behavior of matter under extreme conditions, contributing to our knowledge of astrophysics and the life cycle of stars.