Since I'm sort of a physicist, and have experienced astral projection, and believe its more than "just a dream", I've thought about how it might make sense in terms of string theory and dark matter. I agree its exciting that there's so much unknown in physics theory that leaves room for astral projection. And its exciting that other people are recognizing this.
So I hope you don't take the rest of this as a downer, since the important thing is that spiritual stuff is real and there is so much left to be discovered.
The size of a particle physics 'string' is on the order of Planck length, which is about 10e-35 meters. In other words, take 1 meter and divide it by 100000000000000000000000000000000000. The vibrational frequency of a string will be proportional to its size. If the vibration propagates at the speed of light, that's about 10e38, in other words 100000000000000000000000000000000000000 oscillations per second. The frequency of 'middle C' on the piano is about 16 oscillations per second, and already that's fast enough that its getting hard to feel it as a vibration by touch. And that's already 'higher frequency' than the vibrations most people describe in relation to astral projection. So you can't feel the vibrations of a particle physics string, and those vibrations have nothing to do with the 'higher frequencies' of New Age lingo. Furthermore, if astral matter is in any sense 'higher frequency' than normal matter, you can not feel that as a vibration either. Vibrations of ordinary electron orbitals, for example, are on the order of 1000000000000000 oscillations per second.
One of the main ways 'dark matter' is known to exist, is it's gravitational effects show that its distributed in a spherical cloud around the galaxy, rather than being flattened out in a disk like the visible matter. This implies that it interacts almost not at all with normal matter except gravitationally, and more importantly, it mostly doesn't interact with itself. In other words, it doesn't clump together into forms, aetherial or otherwise. This doesn't fit our experience of 'astral' matter very well at all. And there are other issues, I just gave the first one that came to mind.
Another idea I've had is that astral matter could be sort of a cloud of 'virtual' parallel worlds. But this idea doesn't work either, since we can do stuff astrally that's probably impossible in any physical world. The idea might work for things like teleportation, but not for things like levitating. So all I have presently are a couple of really hazy ideas that don't amount to much:
1. The difference between 'astral' matter and ordinary matter may be more akin to a difference in reactivity than a difference in activity. (I didn't come up with this, it's my interpretation of an image from a dream my sister had.)
2. Providence shapes our world by a process of removing potential outcomes, and leaving a smaller collection of preferred ones, almost like an etch. 'Dark matter' could be involved in this. Spirits have access to a larger cloud of possibilities than what we experience, though I can't say exactly what that means. Their knowledge of this greater meta-reality is part of what allows their understanding of ours.
Maybe what we experience as 'astral matter' when we 'astral project' isn't actually quite synonymous with 'real' astral matter, but its metaphorically quite similar so we confuse the two? In waking life, we have a tactile sense of where our body is, which is essential to being able to move it around without bumping into things. And we typically coordinate that with our visual image of our body and surroundings. These mental models help us predict what is happening, what has happened, and what is going to happen. And they're what we use to make things happen. Suppose there are worlds that are not quite real, but are spiritual models of what can happen, and fates use those to predict what can happen and to make things happen. That seems to me to be a lot like the same kind of thing. However, our mental models aren't always accurate. We can lie to ourselves, or be mistaken. So our thoughts aren't necessarily 'real' in that sense. Maybe the thoughts of the gods are like that too. Maybe the astral is not real in the sense that there's actually stuff there, obeying rules like the Pauli exclusion principle. (I think that one is pretty much essential for something to behave anything like what we might call matter.) So maybe when we try to think of it in terms of some kind of finer matter we have the wrong idea.
I guess it does seem to me though that human imagination and divine imagination must at least in some sense be the same kind of thing, because we do consciously precipitate events in the objective world. We just do that almost exclusively by influencing our own bodies. Off hand I can't see how that can be essentially different, even if the angels do it with considerably broader scope and intelligence.
This is somewhat off topic, but I thought I'd point out that if you have a bright light that is uniformly smooth, the same in all directions and at all frequencies, that's the same as darkness. Energy is about differences. Maybe this thought helps a tiny bit towards understanding what the moon represents spiritually. (It seems to me this follows from Light not being subject to the exclusion principle, which is what reminded me of this.)
Going a bit further off topic, here's the thought I'm working on today, see my dream in the general forum for context if you want to. When we fly in dreams, this is a metaphor for our freedom to move as minds. When I experience some other place or way of being in a dream, I am flying to that place. If I extend my identity to embrace yours a little bit, we fly there together. This freedom starts in our hearts. It wards of the destruction of despair. In life, we are also supported by the comfort of circumstances - food to eat, a chair to sit in, a TV show to watch - and these ward of despair also. But its our will, our movement of our spirits, that gives us freedom to soar to something new or better. Affection, such as expressed physically with sex, is metaphorically like sharing our ability to fly. It expresses that levity as something like a physical support. If we express that freedom or levity in a way that is destructive to our physical conditions, then we have a problem. By 'physical' I mean everything that has a physical basis, seen or unseen, impacting pretty much everything which might be called emotional or spiritual also. Sometimes when we do something to gain happiness, its like taking a higher interest loan to pay off a lower interest debt. We gain a moment of relief, but inevitably we sink lower. We often use sex that way, and use chemistry that way, and even use the affections of friendship that way. We need to use that freedom, our love, in a more intelligent way that nurtures, and builds things up. I'm just brainstorming here, thinking about what I experienced in my dream. Since it ties metaphorically love with the experience of flying, and with moving energy in a dream, it seems relevant to understanding our 'astral' experiences. But not because flying is love. The gears of my mind sort of seize up at this point. From love, all of these other things are possible, yet none of them are love. They aren't love in the same way that buying something with a credit card is not wealth? Everyone knows that none of these things are love, and we can all give obvious illustrative examples that nearly everyone understands and agrees about. At the same time, it seems we're all almost perpetually confused about this. It seems to me like swinging on a swing. You lean back and put your legs in the air, but if you do it at the wrong time it drags you to a stop instead of lifting you up. It seems to me that intelligence is required. And its a global, universal intelligence as well as a personal or selfish one. All of the pieces have to be coordinated. It matters not just where the swing is, but where the rest of the earth is in relation to the swing. So we have to think for ourselves, because only we have the perspective of who we are and what we need, but all the while we need to listen to others also, taking who they are and what they see into consideration. And this has to be anchored in the spirit that ties us all together, and ties our collective human island to the wider reality that we can't see.
I realize this last bit goes pretty far off of the string theory topic, but tend to go overboard as a critic and wanted to try to offer something constructive. And metaphorically it seems related, and attempts to get towards the heart of it all, for me anyway.