The Fluidity of the Universe

I'm guessing that you could consider the creature that some planet stars exert. And I think space-time around really massive objects, like a black hole, will also get dragged around. It seems like there's waves in it and we talk about it as some sort of foam, but what space is the fluid occupying? That's what gets me. So if I think about it, what is a fluid? A fluid will flow and it could flow very slow. Like a river.