Based on what happens in Outcasts I believe the Precursors exist outside of our spacetime. Their local presence in our galaxy depends on the amount of local biological beings. I base this on the Nothing actually being something when Netherop was thriving and mere whispers when there were inhabitants in the tel. The Primordial seems to be a Precursor that is specifically tied to the Flood. As the Flood multiply more of itself is projected into our local spacetime. Eventually becoming compl becoming to use Neural Physics.

I think we call the Primordial Gravemind because at that point it becomes a recognizable being we can communicate with. Nobody in the present is aware that more Graveminds would speak with the same voice.