I have struggled with this issue myself, so I am not sure. However, I think no one is sure here. I don't get this, though.
If the universe is expanding into nothingness, and nothingness equals regular space, which is the universe, how can you say it is expanding? There would be no difference. We would say: ''The matter of the universe is spreading into the universe.''
If the universe expands into nothingness, I think this does not equal space.
Or, the nothingness does in fact not exist/is unavailable, before the universe expands. In other words, the energy/expansion speed creates a distortion in the fabric of space or the space time continuum.
I know it sounds stupid, but that's the only thing I can think of that makes sense.
BACKUP please.