You've run into a confusion, or ambiguity, that English shares with some (but probably not all) other languages. As the term is used in physics, a "sphere" does not actually have any center, as it is the surface of a "ball": The diagram which you've helpfully provided depicts a ball, whose volume does have a center. A sphere has a surface area, but not a volume, and even the description of its surface area is imprecise, because it must take account of that ratio (of the ball's circumference to its diameter) which is pi, and continues into an apparently endless series of variations.
However, even a ball only provides an instantaneous description of the figure (a possible shape of the universe) that you're concerned about, which is not a description complete enough to capture the figure as it's used in cosmology (your first tag), because the volume of the ball may be (and is, at least in that part of the cosmos which is observable by us) expanding overall: Consequently, time must be taken into account.
In Einstein's relativity, time is "visualized" as orthogonal to (in other words, "at right angles to") space: It can only be taken accurately into account through the use of at least two coordinate systems, which greatly complicates the appearance of the resulting figure, known as a "three sphere" or "glome". However, nothing physical like it can ever be seen from any known vantage point, because we do not actually see time: We only see evidence of our passage through it in objects, like the hands of clocks, that have at least some mass.