You probably wonder what this is and expected the answer to be in this post. Well, not quite. I want You to tell me what it is!

It's filling the universe, just like water is filling the ocean. Now, there surely are several theories about this and i want to hear 'em all.

 

What (theoretical) types of matter are there? I'm looking for a term that describes a theoretical unknown matter, something like "x-matter". Is pseudo- or quasi-matter what i'm looking for?

Do you mean Dark Matter?

There is a growing theory that space-time is a superfluid.

http://www.kurzweilai.net/mindx/frame.html?main=/mindx/show_thread.php?rootID%3D61219

 

Seems a bit audacious that we could even hypothesize the "stuff" of space-time but it seems to follow from the data.