These are probably mostly supernova remnants, but also, if you believe the "inside" (i.e. other side) of Black Holes are White Holes, it would make perfect sense to see White Holes in our universe where stuff is sort of "falling into" our universe and exploding outward. There might be an actual hierarchy of these kinds of things so that what we think was our "Big Bang" was actually the location where a White Hole emerged thru which flowed everything in our universe.