Universe
The universe is, in principle, all of material existence or natural things (matter and energy). Therefore, everything outside of the universe is not natural or material, but supernatural (things going beyond nature). However, in order to solve problems with naturalistic explanations of the the origin of the universe, some speculate that there are other universes.
There have been and continue to be a number of competing ideas about the universe, but most agree that the universe is expanding. The size, shape, and age of the universe, as well as how it began, are, however, all points of contention.
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Size
A recent calculation placed the visible universe at about 156 billion light years across.[1] This creates a dilemma of faster than light movement, but it is speculated that the expansion of space itself can exceed the speed of light even if the objects within it moving on their own can not.[1] As an analogy, imagine ants (galaxies), that can't walk faster than 20 centimetres per second, sitting on an elastic cord. Normally two ants moving away from each other could not exceed a speed of 40cm/s relative to each other (speed of light). However, if the cord is stretched while the ants are moving (expansion of space), the ants' speed relative to each other can be greater.
Current theories on the universe means that scientists are only able to account for 4% of the matter in the universe, so new theories of dark matter (unseen matter) have been developed to explain this.[2]