@jeeprs,
jeeprs, great questions...
I have a few theories of my own, perhaps not derived solely by myself but derived by my research which gave me the ideas. Here are my ideas;
There are two ways to blow up a balloon. One way is to supply air internally which will allow the balloon to fill and expand. The other way is to place the balloon (tied closed) into a container and vacuum out the air surrounding it which will lower the pressure inside it and give the appearance of inflation.
So why did I give you this visual? Well perhaps dark matter isn't contained inside our universe but instead, outside it. So our universe isn't pushing the expansion but instead the expansion is a pull from dark matter outside the universe. This would be consistent with it not being observable nor consisting of atoms yet still impacting our universe.