How is the Universe Expanding?
Before considering what the expansion of the Universe is, it is helpful to know what it is not.
You might imagine that, in our expanding Universe, all of the galaxies, gas clouds and other objects are flying away from each other in all directions. This is not the case, however. Strange as it might seem, the Universe is expanding, not through any movement of its contents, but by expansion of the empty space between them.
One way to visualize this is to think of the galaxies and other objects as points drawn on a balloon, the surface of which represents the Universe. As air is blown into the balloon, the size of its surface increases, pulling the galaxies further and further apart. Alternatively, in three dimensions, imagine the Universe as a plum pudding in the oven. As the pudding bakes, it grows in size and the distances between the pieces of dried fruit increase.
A problem with both of these illustrations is that the model “Universe" is expanding into something – the air. This is not the case for the real Universe, which is not expanding into anything for the simple reason that there is nothing beyond it to expand into.