What is beyond the observable universe? What is beyond the edge of the observable universe? Scientists say that the Big Bang is the cause of all creation in the known universe, but it would seem the bang must have happened in the center of the universe ?
What is beyond the observable universe?


There's no way to know because we can't see it.  No information from the unobservable part of the universe can ever get to us.  If we can't see it, we can't know what's there.

Over time, the observable universe grows and we see more of the universe, though this is actually a pretty complicated due to the rate of expansion and I'd rather not open that can of worms, but for further reading, see this question or this one. 

But avoiding the complexities of dark energy expansion, we have a pretty good idea that the unobservable universe is very much like what we can see.   This makes logical sense because it likely formed the same way at roughly the same time and out of the same stuff.   So, we expect it's just more galaxies and more space.    

In addition to this, if there was a point where the galaxies stopped being there at a theoretical 'edge', not far from the edge of the observable universe, we'd expect to see that lack of gravitation resulting in the galaxies moving away less fast and that's not observed.   The galaxies appear to be moving away from us equally fast in all directions, so we have good reason to believe that the universe is uniform out considerably beyond what we can observe.   It's at least several times larger than what we can see.

If we think of the big bang at happening at a point, which is often depicted in sciency-tv shows, then we shouldn't expect uniformity due to gravity and faster moving parts spreading out more, but what we see when we observe the universe is uniformity.   That means that if it did explode from a singular point, it must be so large that we can't see any variation.

The more popular approach is that the big bang didn't happen at a point but it happened everywhere.   Google Everywhere Stretch for some explanations of this.   

It's worth noting that not all shapes (or manifolds) have centers.   The surface of the Earth has no center on the surface.   So even if we could see the entire universe, finding it's center might still not be possible.  

Determining the center of the observable universe, however, is easy.   We can see in all directions just about equally so we're the center.  But we can't get a good grasp on the size and shape of the entire universe, or just universe is more correct.   Since we can't see all of it, finding the center is impossible.


  The bang must have happened in the center of the universe.


Keep in mind that the big bang isn't well understood.  It's what physicists call a singularity which means, the math breaks down.     The point exploding outwards is often represented, but shouldn't be considered accurate.   

Mathematical approaches can come up with a variety of possibilities like hyperplanes intersecting or 4 or 5 dimensional spheres.   (Stephen Hawking had a 5 dimensional sphere argument where no big bang happened, it's just all a circle - that's in one of his books, I think blackholes and baby universes).   He was able to create a universe without a big bang.   That the universe is infinite and the big bang happened across infinite space is also a popular idea today.    The singular point that blew up into the universe isn't big right now, though that viewpoint still applies to the observable universe, but it's fallen out of favor for the entire universe.

My two cents anyway, as a layman.