From Destinypedia, the Destiny wiki
Alkahest, as it was named by Clovis Bray I, is a substance produced by exposing Vex radiolarian fluid to the paracausal influence of the Darkness. It is an essential component in the creation of Exos.
Being derived from Vex mind fluid, Alkahest has similarly versatile computational properties, but lacks the virulence of raw radiolarian fluid. As a result of its alteration by the Darkness, it also has paracausal "anti-structural" properties, which allow it to disrupt the "loop/billboard/crash" cycle that plagued early exominds. Its presence in every Exo's cognitive hardware also causes Exos to be inherently sensitive to paracausal influence, such as messages from the Darkness.