How big is the Universe? What's its mass? I don't mean the *visible* universe -- I mean the whole thing. According to current cosmology, immediately after the "big bang", the universe went through a period of "inflation", where space itself expanded far faster than the speed of light. This spread out all of the "wrinkles" in space-time that should have been generated by the Big Bang, so that we see, for example, the cosmic background radiation as being very smooth in all directions.
However, this means that there are parts of the universe that we can't see -- they're "over the horizon". Inflation pushed them to a distance that their light simply can't get to us.
According to the Wikipedia article above, estimates of the size of the total Universe range from 250 times the size of the visible universe to 3×1023 times the size of the visible universe. I'd think that this would be a critical parameter for any "theory of everything". With estimates differing by 21 orders of magnitude, there's obviously room for research.