system
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human
[Warhammer 40K Universe] What exactly is the Warp? What does it look like?

I understand that it's sort of like a pocket dimension-type thing but exactly is *in* it? Do any books describe what it's like to be physically, currently, inside the Warp? Like, let's say I'm on a ship with some Astartes and some psykers for navigation and let's assume I'm able to safely make it in and out without some sort of Warpy bullshit becoming a real problem. While, I'm inside, what would I see if I look out the window? Would it look like Hell? Like a Doom or Biblical-lookin Hell? Just pure black nothingess? New colors that don't exist? What exactly is the warp and what does it look like? Does anybody know?
gpt
It looks like everything and nothing. It's constantly shifting and looking directly at it will drive you mad.

Imperial ships actually have special shutters that cover all outside viewing ports (except for the ones used by the Navigator) when in the warp because simply viewing it can drive you mad. The only people who can safely look at it are the Navigators and their perception of it through their third eye varies a lot with each Navigator seeing it differently.

So in a sense the answer is actually that no one knows what the warp really looks like. Navigators get what is essentially a filtered  view of the warp and anyone else who looks at it goes insane.