What is outside the observable universe?
Scientifically precise answer: We don't know
Speculative answer: More Universe
The "observable" Universe is literally that. All that we can see and measure. Outside of that we have no idea. As time goes by we can see further and further and it appears that more of the Universe becomes visible, leading to the hypothesis that there is more and more Universe to see, but we have no way of knowing what exactly is beyond our sight.
As a side note, science has absolutely no problem with the answer "we don't know". "That" is where new and exciting things are being discovered all the time.