An aethyr is any spatiotemporal emergence initiated by consciousness which should be differentiated from mere imaginings which are not exactly the same. The two most recognized would be the macrocosm and its' inferior microcosmic reflection, the dreamscape.
These aethyrs appear to have material substance, but this is only illusion. The nature of these can be extrapolated to that of mind, or, consciousness. It is difficult to assess the exact nature of primordial consciousness, although a 'ballpark' can be achieved in comparing the emergent properties of the macrocosm to that of the subconscious dreamlike properties of its' microcosmic counterparts.
In both respects, as would be expected. There appears this mysterious acausal parameter that initiates consciousness followed by these emergent properties. We don't know the when, why, or how we will dream, just as it is unclear why there even is a universe, as opposed to a static eternal state of nothingness, only that 'something' happens.
With 'time' being a factor, and with every passing moment, however small, there is a thought of these 'past aethyrs', and whether or not they exist right now, at this very moment. These aethyrs being bounded by negative existence. There is nothing beyond them, neither time nor space. They are everything, there is nothing else, neither would there be, how could there be. How can they even be, only God knows, or perhaps even not.
When there is nothing in existence that is external to the primordial, then this expansion of space, or, consciousness, is understood to occur within. This is what is meant by the expansion of the 'something' into the 'nothing'.
And although this may be referred as 'illusion', it is only illusion of perception; on the contrary, it is quite real; to paraphrase Otis if I may "It's as real as I want it to be."