What makes dark energy "dark"?
a. its unknown nature
b. its completely black color
c. its existence beyond the visible spectrum
d. its inevitable creation of a totally destructive big crunch
nucleosynthesis began when and ended when
a. the big bang occurred; the big crunch occurred
b. nuclear reactions could first occur; matter had condensed into galaxies and stars
c. the cosmic microwave background radiation formed; dark energy replaced it as the dominant energy form
d. the universe had cooled enough for atoms to combine; protons and neutrons were too far apart to form nuclei

Dark energy makes "dark" the answer is option A. its unknown nature.

Nucleosynthesis began when the answer is option D. the universe had cooled enough for atoms to combine an ended when the answer is option D. protons and neutrons were too far apart to form nuclei.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.

The answer is d) by requesting advice on science internet sites