one possibility is that all of the atoms in the absorbed matter contract by letting their electrons, which occupy most of an atom's volume, combine with and neutralize the atom's protons, resulting in neutrons. without it's electrons, each atom occupies much less volume. as time goes by, the black hole absorbs more and more matter, continually compacting it into neutrons and increasing in mass. if you own or command a starship, try to steer clear of black holes ( :