Neutronium is an unobtainable resource in Oxygen Not Included, usually hidden around the corners of the game's map. It has an unlimited mass and cannot be dug without the use of cheats. This object is unknown, but probably assumed to be used as some sort of barrier for the edge of the map.
You can crash the game if you manage to bypass the barrier, behind this so called 'barrier'. There is probably two barriers to prevent people from going any further. Building or digging anywhere else outside the barrier crashes the game as well, so don't try it.