Hi SM

The Dark Universe - if it exists, because it won't if MOND is true - maybe made of shadow matter, which is just like regular matter but with a different electromagnetic and nuclear force. It interacts with our world via gravity and the weak force, hence why it mostly indetectable.

Another possibility is that dark-mass is regular matter in another part of hyperspace hence we feel it via gravity and nothing else.

As I understand it ultra-heavy nucleii reach a point where they start losing energy in a process akin to Hawking decay of black-holes - they polarise the vacuum so much it starts leaking away mass as positrons ripped off virtual particle pairs which have lost an electron to the nuclear protons.

Dark matter could be so many things that the physicists are having a hard time deciding what they can distinguish experimentally.

Adam