In addition to colliding galaxies, we (conclusively) know they come from supernova, as measured from Sanduleak 202 (SN1987A). The energy they carry obviously indicate the incredible magnitude of the "event" of their origination, and quite obviously the big bang must have generated a lot of them. I wonder since they are mostly unaffected by gravitation and travel in straight lines from their source, do they ultimately leave the universe? Or do they somehow 'bounce ' off the bubble we call our universe and dissipate? Maybe their existence provides additional proof of an expanding universe. Or could some ultimately come from another universe?