1. What are the most abundant elements in the universe?

2. What effect does dark energy have on the universe?

That is for question 2, Dark energy and dark matter seem over analyzed, although they are no different than gravity and wind. Dark energy can be the pulling caused by black holes, stars, and large planets. Dark matter also is just the effect of cosmic dust and debris pushed from explosions but also pulling caused by distant gravitational bodies. : D

Hydrogen is the most abundant element in the Universe; helium is second. However, after this, the rank of abundance does not continue to correspond to the atomic number; oxygen has abundance rank 3, but atomic number 8. All others are substantially less common.