Universe being flat and why we can't see or access the space "behind" our universe plane? I'm a layman, but i watched some intereting videos about big bang on youtube[michio kaku, hawking this kind of things, not some crackpots :)] 
I described everything on my picture: 

So is there any possibility to see that inner side, for example, by creating wormhole, or entering new dimension? Or i get something wrong, on the way, and my model is totally wrong?
And another question. I heard that even if you would fly a ship across the universe you would eventually end in the place where you started, because we live in a 3d space that is defined on a surface of big sphere. But why a big sphere? Universe might be a big mobius strip, and the same principle of ending at start will apply. Is it possible?
Thanks in advance, and please don't bash me for silly questions. Im just curious, but i don't have any degree in physics. Im also not an english native speaker.
 A: When you're talking about curvature it's important to distinguish between intrinsic and extrinsic curvature. I struggled to find a good summary of the difference: page 6 of this PDF discusses it, or Google for similar articles.
A very common analogy for spacetime curvature is the rubber sheet, and this is an example of extrinsic curvature. We take the two dimensional sheet and deform it in a third dimension to create the curvature. With extrinsic curvature it's entirely reasonable to ask if we could move in the third dimension to see "behind the sheet".
However the spacetime curvature described by general relativity is intrinsic not extrinsic curvature and is not caused by bending the three dimensional space in some hypothetical fourth dimension. So you can't move out of spacetime to see what's behind it.
It's difficult to explain intrinsic curvature, but let's try by going back to the rubber sheet. Suppose you keep the sheet flat, but you grab the sheet at a point and pull the sheet in some direction:

The grid is supposed to show how the sheet has been stretched. The sheet is still 2-D, because we haven't stretched if upwards or downwards, but it's been deformed so that the spacing between grid lines changes. The key think (and the hardest to understand intuitively) is that for Flatlanders living on the sheet the grid lines still look straight. A Flatlander walking along the centre vertical grid line would think they were walking in a straight line, but would actually be moving in a curve.
This type of curvature is what happens in general relativity. It's intrinsic not extrinsic. So to back to your question, you can't move behind the universe because there is no behind to move into. There are only the three spatial and one time dimensions - it's just that they are intrinsically curved.
A: As far as we know right now, you cant see outside the universe.  For that to be possible you'd need some way for information to leave and enter the universe.  I'm not aware of anything in physics that allows that.  Even in black holes, to the best of my knowledge information and energy are preserved once they go inside.
To the second question: If the universe is "closed" then yes, in theory you can travel in a line and get back to where you started.  But, if it's flat(like a plane) then it would be infinite and you just keep going forever.  We just dont know for sure what the geometry of the universe is.  It's very likely that it's flat and infinite.
