A good explanation of the expansion of the Universe is to imagine space as a balloon. Everything in Space is on the outside of the balloon, which is, in itself, pretty one dimensional (flat). Try drawing a few dots on a deflated balloon to represent stars, planets etc. As you inflate the balloon, all points on the balloon get pushed further apart. The action of inflating the balloon represents the Big Bang. Everything moves outwards away from the centre of the balloon and hence futher apart. So Space can be viewed as an inflating balloon of unknown elasticity. Short cuts such as wormholes would be represented by a passage from one side of the balloon to the other via the central cavity. Of course, nobody knows what is beyond the expanding universe, though presumably the partial vacuum that we call the universe is expanding into (even being sucked onto?) a total vacuum surrounding it.
I have a bit of a problem with the theory that everything in the Universe originated as a piece of matter the size of a football.