Unconditioned Dharmas

In reviewing the course material in detail this last week, I'm becoming more curious about the various schools' theories of unconditioned dharmas.

Both the Mahisasaka and Sarvastivada include space as an unconditioned dharma.

I'm curious as to how they arrived at that position, if there were cultural or other religious groups' influences as well as what their concept of space was.

Clearly as we now know space it is not "empty" and could not be considered unconditioned.

But I'm wondering if this concept of space as empty is related to later notions of emptiness in the Perfection of Wisdom sutras and the Madhyamaka school.

Perhaps this is covered in ME6205 Survey of the Doctrines of the Abhidharma Schools.

Can anyone recommend a resource for digging further into this point of philosophical analysis?