Question

What is behind the universe ?

Answer 1

Some ideas propose the Universe to be, literally, infinite in all direations - no matter how far you go, you'll /never/ get to the end, and you'll /never/ get home again. It just goes on, and on, and on, past any sane human reckoning, at the speed of light, forever.

Some other ideas propose the idea that the Universe is a great hypersphere, and we're just residents on its 3d "surface" or "brane", as another idea calls it - imagine it as like bacteria living on the surface of a ball. It's infinite, but if you travel far enough you'll get back to where you started.

Another idea proposes that the universe condensed out of a primal Chaos, and one day it will, like a soap bubble, dissolve back into that formless Chaos. That's the Fiat Nox hypothesis of Peter J Carroll.

But there's one other idea I'll describe here for you.

Imagine that ball I talked about earlier, floating in suspension in a sea of other balls. All these balls are of different sizes, and more or less what lies between these balls are more balls, smaller and smaller, each containing a whole universe unto itself, like frothy carbonated water. There's no medium between them - just more balls, formed from branes, a fractal storm of them, each ball being a full size 'verse, regardless of its apparent size "outside" it.

And that multiverse, that suspension, goes on forever, in directions humans can't even begin to fathom, possibly beyond what we could sanely consider "forever". Just endless 'verses, all packed together cheek by jowl.