What is it expanding into? Will it ever stop? Will we ever be able to go outside the zone it is expanding into?
nothing, no one knows, and no
The concept of the makeup of the universe is extremely confusing for most people. First of all, it is generally agreed that the universe is ball shaped. However! There is no interior and no exterior. There is only the space between stars and galaxies. As the universe expands, it creates space but it is not expanding "into" anything. One way to imagine the universe is to imagine a partially full balloon. The surface of the balloon, and only the surface, is the universe. As the universe expands, it creates space and therefor more distance between the galaxies.
the universe is expanding into what was nothing, no the expansion will never stop, possibly if we cease to be material beings.
Space and time are both created by the expansion of the universe. The equations of relativity indicated that before the expansion of the universe started neither space nor time existed. As the universe expands more space is created. What existed in what becomes space due to the expansion of the universe as far as is known is nothing.
The supernova experiments of 1997 and 1998 proved that the rate of the expansion of the universe is increasing. This means that the expansion of the universe will continue until there is effectively nothing left but "empty " space.
Matter can only exist within space time. To go outside the space time continuum would require become immaterial