No one can say. But as it’s gravitationally interactive, one would expect to see huge globules of the stuff crashing into visible bodies throughout the cosmos. Yet no such interactions are seen. Thus it seems but a device to explain something not yet explainable.
“But as it’s gravitationally interactive, one would expect to see huge globules of the stuff crashing into visible bodies throughout the cosmos. Yet no such interactions are seen.”
Obviously, it must be a nonluminiferous aether so it can project a gravitational field but can’t physically interact with regular matter!
/sarc