The simplest explanation considered for the accelerating expansion of the universe is: a) planet explosions. b) supernovae. c) stellar winds. d) green energy. e) a cosmological constant. 49. After the dark energy (whatever that is and assuming it's not just an effect of a true cosmological constant), the most abundant form of mass-energy in the universe is apparently some form of matter known only (at least to circa 2020) through its gravitational effects. We call this matter: a) luminous matter. b) dark matter. c) baryonic matter. d) invisible matter. e) mirror matter. 50. In Big Bang nucleosynthesis, the two most abundant products are: a) hydrogen and iron in about a 1:1 mass ratio. b) hydrogen and helium in about a 3:1 mass ratio. c) hydrogen and helium in about a 1:1 mass ratio. 10 d) hydrogen and iron in about a 3:1 mass ratio. e) helium and iron in equal amounts by mass