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This sounds like a seductively simple question, but it is virtually impossible to answer! Intuitively, one tends to think that it must be spherical, because it started to expand "from a point in space in all directions". Not quite.
Our observable universe is very close to a sphere with us at the center, but that is only because we have a spherical horizon around us. The horizon is roughly at the distance that light could have traveled since the birth of our Universe. Shown below is a small piece of an "infinite lattice", but only observable up to where light could have reached us since its creation. Look down the "shaft" at the bottom-left and you will see the "horizon" there (too much structure in most other directions to notice it).
"Infinite lattice"(1) 
This so-called particle horizon is somewhat like the horizon on the open ocean, which appears at a certain distance (radius) from us along the surface of the ocean, depending on the curvature of the Earth. Does that mean that the ocean is circular? Obviously not! The universe can also not be taken as spherical just because we happen to observe a spherical horizon.
Like the total ocean, the Universe at large may have any of a number of different shapes, with the distinction that the Universe may even be infinite, in which case it has no shape at all. This is a sobering thought – our 14 billion light-year radius observable universe may only be an insignificant spec in an infinite "sea" of space and matter-energy!
Present observations tend to fit a model that is infinite in size, but (perhaps significantly), with a very slight statistical bias towards a positive space curvature. This implies that it may be as large as can be without being infinite, tending towards being spherical. Here spherical is not what it means in the usual sense of three dimensions (like Earth), but in at least four dimensions. Such a hypothetical thing is called a four dimensional hypersphere. Rather don't try to visualize it!
In representations however, it is possible to discard one of the normal spatial dimensions and replace it with what is called a hyperspace dimension. All Earthly observers are now two-dimensional beings, the third dimension belonging to hypothetical "hyper-observers". Shown below is a semi-sphere (3-dimensional), with its surface area (2-dimensional) divided into four equilateral triangle quadrants. We are hyper-observers, floating around in hyperspace...
A "hypersphere"(2) 
The inside angles (each 90°) of each original triangle add up to 270°. However, keep on dividing each triangle into further equilateral triangles and the inside angles of smaller and smaller triangles will eventually tend towards adding up to ~180°, just like on flat Euclidean space. Put differently: make the sphere larger and larger, with more and more subdivided triangles, and the smallest (still large) triangles will tend towards being flat.
Our observable universe may be somewhat like one small triangle on a very large hypersphere, looking locally flat, but in reality having the same curvature as the universe at large. In three dimensions, our observable universe is not triangular of course, but spherical. The triangles are just an imperfect analogy, serving to illustrate the point. Let's pretend that our universe is such a huge hypersphere.
If the universe was not expanding, light beamed into one direction would eventually circumnavigate the hypersphere and return to its origin from the opposite direction. However, since all indications are that the universe is expanding at an accelerating rate and is "near infinite" in size, this scenario cannot happen. Light beamed into one direction would never return to its origin, so being spherical has no significance whatsoever! For all practical purposes, the universe can be taken as infinite and that's that.
There are some other interpretations of the data, but the evidence is stacking up against them. I will deal with a particularly interesting one in next week's post. It's called a "soccer ball shaped" or, more accurately, a dodecahedron universe…
Notes:
(1) Figure credit: Coburt Jordaan, from a CR4 post on Escher's infinite lattice.
(2) Figure credit: Relativity 4 Engineers, from a chapter freely down-loadable from here.
-J
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