Q: WHAT IS IT EXPANDING INTO?
A: there is an expanding sensation happening. but it's not clear how to define or explain that. everything seems to be getting further away from each other. but then again, the earth feels like it's accelerating up under us, but that would be backwards for the other side of the planet,... and it's not. So there is clearly something relativistic going on in both cases that is possibly a next level of whacky that no one has quite nailed down yet. but these things always get found and learned eventually. even the weird ones