It is absolutely certain that we are only seeing part of the universe and, if the inflationary theory is correct, a tiny fraction of it. This is a consequence of the finite speed of light and finite time since the big bang. We can’t see anything whose light would have taken more than 13.7 billion years to reach us. Every year, we can see one light year further.
If the universe is spatially flat, then it is probably infinite, which means the part we can see has measure zero and there is infinitely more we can’t. There is no reason to believe the parts we can’t see are anything like what we can, but there’s no way to know if this is true or not. In his book Our Mathematical Universe, Max Tegmark calls this the “level I multiverse” which I described in my review as:
The level I multiverse consists of all the regions of space outside our cosmic horizon from which light has not yet had time to reach us. If, as precision cosmology suggests, the universe is, if not infinite, so close as to be enormously larger than what we can observe, there will be a multitude of volumes of space as large as the one we can observe in which the laws of physics will be identical but the randomly specified initial conditions will vary. Because there is a finite number of possible quantum states within each observable radius and the number of such regions is likely to be much larger, there will be a multitude of observers just like you, and even more which will differ in various ways. This sounds completely crazy, but it is a straightforward prediction from our understanding of the Big Bang and the measurements of precision cosmology.