4D
By analogy, the universe of a 2D (two dimensional) being is a plane, but this plane may curved into a sphere.  This means that if a two dimensional object goes forever forward in a straight line, the object will end up in the same position it started from.  So, applying this idea to three dimensions, we can say that our universe is curved into a fourth dimension.  Thus, if a three dimensional object walks forever in a straight line, the object will end up in its starting point.