The two biggest mysteries in cosmology are what is dark matter and dark energy made of. I'll theorize that dark matter is the residue of matter and anti-matter annihilation that does not interact with itself or our matter very much but it does interact by way of gravity.

The second mystery is dark energy which is making the universe expand with acceleration. My best guess is that black holes and dark matter create new space in a similar way that mass can convert to energy. In the incredible energies of black holes or dark matter black holes, mass is converted to space.