That, or interaction with the gravitational field. Neutrinos from the supernova traveled through essentially flat spacetime, far from any masses.
How about the mass of the supernova from which the neutrinos had to escape?
"...no serpent, no apple, no fall that toppled man from a state of innocence..."
Come on, you can do it, just one step further... -> No God!
"It ain't over until it's over." -- Casey Stengel