Is the universe simply a mass of atoms in 'void'; what does it look like?

No, beyond the universe does not lie anything. Not even space itself. You cannot be outside the universe because there is nothing to be in. The three (or four) dimensions you think of as space, do not exist outside of the Universe. According to the Big Bang theory not only matter was created during the big bang but most importantly space was created during the Big Bang.

From CERN:

There's another important quality of the Big Bang that makes it
unique. While an explosion of a man-made bomb expands through air, the
Big Bang did not expand through anything. That's because there was no
space to expand through at the beginning of time. Rather, physicists
believe the Big Bang created and stretched space itself, expanding the
universe.

That is also why an inflationary phase where space expanded faster than the speed of light was possible. Matter cannot travel faster than the speed of light, but space can expand faster than the speed of light.
So it is possible for two galaxies to 'appear' to have a relative velocity larger than the speed of light because in actual fact the space between those galaxies is expanding.

As to the shape of the Universe: You can only assign a shape to something relative to the space it is in. As there is no space outside the Universe, the answer to your question would be that the Universe has no shape.