One of the hardest things to do is to try to visualize what the universe might look like on its largest scales. The only guide we have is Einstein's theory of general relativity, and until such time as this theory is shown to be incorrect, it is our only best shot at visualizing what is going on.
The universe does not exist, embedded in some larger space, but contains 3- dimensional space within itself. This isn't philosophy, but hard physics. The universe is not an expanding sphere in space, nor is it a torus or any other shape in space. Instead, it is the space itself that for a closed universe is bent around into a finite spherical volume. At any instant in cosmic time, the shape of space in a closed universe is that of a hypersphere with a finite 3-dimensional space. At future instants in time, the radius of this hypersphere increases or 'stretches'. The larger picture is that of a 4- dimensional hypersphere, and it is this 4-dimensional shape which is a solution of Einstein's relativistic equation for gravity. Newton's equations just don't work when you talk about curved space and gravity.
When you take the 4-dimensional hypersphere and slice it like an apple, you are slicing it along some unique instant in time. The cross section is a 3- dimensional spherical volume just as the slicing of a 3-dimensional apple gives you a 2-dimensional cross section. To visualize what the 3-dimensional slice through our universe is, is a bit tricky and that's where you end up with all types of misunderstandings. This slice is not a balloon-like object. It is a mathematical object resembling a balloon whose surface is not 2- dimensional, but 3-dimensional space. The radius of the balloon does not represent a direction in space, but a scale of the curvature of the 3- dimensional surface 'in time'.
No one ever said that understanding the universe would be easy for beings that live on a small planet, and who never travel more than a few thousand miles from their birth place!