Where we are, and in the conventional galaxies etc. that we see, there is a whole lot of relatively uniform space but in isolated places within that space, there are pockets of concentrated "mass" which is highly organised into all the objects we observe and study, and out of which we are made.
Now suppose that there were regions of space where that is all turned on its head, and there were huge vast swathes of relatively uniform matter, and within that matter there are pockets of "space" which is highly organised into some other form of object, out of which other objects and beings are made.
Obviously on the first, and simplest test - the question of "how much" dark matter there is, this theory passes the empirical test of reality with flying colours, i.e. astronomers seem to observe much more mass in the regions of hypothesised dark matter in accordance with what this conjecture implies.
What other testable predictions does this conjecture make, and how well do they fit our observations?