Phlogiston

A volatile, flammable substance that exists outside and between crystal spheres[1]. Appears as an infinite storm of swirling colors. Spelljamming ships fly through it, but must not have even the smallest fires lit onboard, lest it cause a devastating explosion with the phlogiston.

Some liars and fools call the Phlogiston "the Astral Sea" and claim it is part of or identical to the Astral Plane. This is unequivocally false. The Phlogiston is entirely on the Prime Material Plane.

A map of the Phlogiston: http://nerik.orpheusweb.co.uk/files/Spelljammer/Flow_map_01-06-12.svg

Illithid raiders in the Phlogiston