Dark matter. Im not sure if said a bit about it in this.
Maybe the article might interest you anyway seen as your curious about this. But there might not be much about it.
Dont feel the need to click on it beacause its there as im pretty sure it's dark matter. We of course dont yet understand it. The theory as far as I know is that it's sort of like a field of energy. Smaller than that of atoms or hadrons (protons + neutrons)
Very inquisitive question by the way :)