[Sephiroth]

If the elementary particles that are generated by collisions in supernovae (or something) create replacement matter, which is one of the theories going round somewhere, then we have the answer.

I don't think that's such a big "if". Neutron Stars, for example, when they explode, Neutrons break apart; particles collide and stuff is formed. Obviously something has to happen when particles collide. Seems logical to me.