Can you give some references for that “heavy elements ejecta would end outside of the galaxies” please? I’m interested because I understood that the dark matter is mostly away from the center of the galaxy, and isn’t then that the dark matter kept heavy elements “still in the galaxy” only on the outer parts of our galaxy, and can it be that most of heavy elements do remain in the galaxy where it’s “thicker,” that is, closer to the center? I can imagine that then the problem is just that the stars closer to the center tend do provide more hostile environment for life as we know it. That would then also contribute to the possible “rareness” of the life, and that again Earth was on the “just the right” place.