A Core Element is a novel type of matter which can be found in the debris of advanced Dark Fog. It has been discovered in experiments that a Core Element can effectively and spontaneously combine itself with other types of matter, but it can't be singularized by Strange Matter. Therefore, adding a Core Element to the singularization reaction of Strange Matter can slow down that reaction and make it more orderly.
Dark Fog Debris
This material is obtainable by killing Dark Fog units that are level 24 or above and having it in your inventory unlocks the hidden research High-Density Controlling Annihilation, allowing you to unlock a vastly more powerful fuel source. Additionally it is a component used to produce that fuel source.