Wow, that's new to me. My understanding has always been that space is just the absence of matter. The ideally of null/void expanding is strange, to say the least.
Do you have recommendations for any books or resources online that break this down, and explain it in an ELI5 way?
I don't know any good resources/books on the topic, I've learned most of it via the education system with some additional background in internet.
There isn't much of a good analog from daily life, but essentially as you said, the null/void itself is becoming more. Two objects with a certain distance between eachother will find this distance growing even if they are not in relative motion in reality, though from either's perspective, the other is moving away at increasing speeds (due to more space inbetween expanding). Eventually the distance will be large enough that the space created between the two objects will exceed the distance that light could have travelled; the object becomes superluminal, or faster-than-light. This is allowed because it's not actually the object moving but the coordinate system they are in moving. With some luck you can still observe these objects.
Do you have recommendations for any books or resources online that break this down, and explain it in an ELI5 way?