The neutrinos from SN 1987a , after traveling over 100000 light years , were only delayed by seconds if at all. A neutrino can weigh no more than 1.1 electron volts (eV)—less than one-500,000th the mass of an electron. Since the energy of these neutrinos was in the millions of electron volts they had to be traveling very close to the speed of light to have that much energy. The models of supernovas predict 99% of the energy of the supernova is carried away by neutrinos.
It is my prediction that most of dark matter is merely neutrinos that have slowed down somehow. The universe pumps these particles out in huge amounts with almost nothing capable of removing them.