Where did the space come from that the universe expands into?
The simplest answer is that the space between galaxies 'dilates' and does not come from anywhere. Another way to think about it is to imagine the universe as a kind of watermelon. When sliced perpendicular to its long axis, we can marvel at how the cross sections of each slice begin from a small wafer, expand to a maximum area and then diminish in size as we reach the other end. General relativity says that the universe is a 4-dimensional object, and that gravity can only be described in its full 4-dimensional terms. This is hard for us to do because we keep wanting to think of things happening in space at separate times. We insist on slicing gravity's field one time step at a time. At each time step, the volume of space increases, but in some sense, this space, like the pulp of the watermelon, has always been there. In each slice, the space does not 'come' from anywhere, it is just revealed to us a moment at a time like the pulp in the slices of the watermelon. Space seems to appear out of nothingness, but in its full context as an object existing in both space and time, space has always been present and isn't arriving from anywhere.
