Mindscape

"The Mindscape" is an interdimensional location separating the real world and the nightmare realm, and is also the realm where Bill Cipher and Ford Pines first met.

History
Bill first mentioned the mindscape while explaining to Dipper that he was stuck in there, being pretty much a ghost that couldn't be seen or heard without a vessel.

While Ford was sleeping under a tree, he suddenly found himself in the mindscape, where he checked some files before finding Bill himself. After making a deal with him, building the portal and then having Fiddleford find out about Bill's true plan, Ford went back to the mindscape to see what was going on, getting a peek of the nightmare realm in the process.

Appearance
The mindscape is a vast, space-like place with hundreds of objects flying/hovering aimlessly around it.