Of course after considering the two-dimensional balloon surface expanding into the "third dimension"...
...we must then try to imagine a three-dimensional space expanding into the "fourth direction"...
Since the local speed limit imposed upon matter is speed c, then all matter must Locally "fly" with speed c into the "fourth dimension". Since the size of the universe is so extremely vast... it would seem as though the matter within our imeditae vicinity is not diverging as the universe expands radialy outward. Me, the room I'm in, the Earth, and the stars in this galaxy... are all "shooting" into the fourth dimension with pracitically parallel trajectories... therefore we all "stick together" within the local frame.
However, when looking upon distant galaxies, we begin to notice the pattern of divergence. Everything appears as though it is being pulled apart on the global scale... and yet "sticking together" on the local scale.
Nevertheless we all have the same local frame speed of c into that fourth direction. The velocities of objects can be diverted to motion through different directions, but the ultimate speed remains unchanged (speed c). So whether we travel into the "fourth direction" or we travel somewhat askewed into our normal "three directions" is a matter of "tipping" our frame's "light-cone" into a designated direction (becomes apparent as normal "motion" or "speed").
Thinking four-dimensionaly, and in terms of expansion... you begin to realize the possible origin of "time". As Luis and I argued with Alex before... "we move with speed c through time" and "time is the fourth-dimension".