The Stargate in Stargate, Stargate: SG-1, and it's derivatives, creates a wormhole between two Stargates, which most of the time has an unstable vortex kawoosh that tends to destroy anything that's sufficiently away from it's surface, before falling back in and leaving a mostly flat water or pool like surface. But what is the surface made of? It tends to ripple like water, but it's obviously not water. And in the season 2 episode "A Matter of Time", a large gravitational force of a black hole on the other side was enough to make it spin down/inwards like an eddy or whirlpool/maelstrom/vortex.
In-Universe answer please.