Density is a tricky thing, a jungle can look to a human as something that is hard to pass through, while for an butterfly it is nothing but wide open spaces. The same point of view comes into play when we want to consider the density of Aether.
Imagine that we could zoom-in to the smallest particles observable, than we will notice that even the smallest things are build up out of multiple things and that the space we thought was empty, is full of small particles.
In a jungle, a butterfly can move from one place through an other, by flapping it's wings, pressing-down on air molecules that we can't observe, it more or less walks through the air, and atmospheric-air-density defines what size a butterfly can be and how fast it can fly, there is a perfect balance between the construction of the insect and its environment.
If we want to know what Aether and matter are made of, than we'll have to search for a similar balance. The chaotic movement between the smallest particles will define the pressure/density of the Aether-gas and it will also define the rigidness of the currents within.
Some particles in the Aether, will streamline behind others and form groups. When the leading particle of a linear group collides with a single particle, coming from an other direction, than the 2 colliding particles will move out of the way and the group can continue moving forward, the blast of the collision will cause the first particle to slow down a bit, and the group will become more tight (bonding). Linear groups can curl up and become a unity, paths will appear where particles start to 'slipstream' and form trains. The closer particles are the more significant orientation of rotation is. Parts of the group can work together or against each other, generating harmony or friction. Friction will cause the structure to break apart, harmony will generate solidity.
At the end of this level of formation, solid gear-like structures will emerge, that are made from synchronous rotating units, who have the ability to keep on rolling through the Aether, without being slowed down by the density of the Aether. Only structures that have the same density will be able to stop other structures. At a next level, these units can group in such a way that they generate larger current-streams within the Aether, attracting or repel other bodies, just like propellors.