The correct match between ideas related to the formation of the universe is:
structure forming in the early universe — dark matter
This is because dark matter is thought to play a crucial role in the formation of structures in the universe due to its gravitational effects, even though it does not emit light and is thus invisible. The other options are not accurate matches. Baryonic matter represents only a small fraction of the total mass of the universe, dark energy is associated with the expansion of the universe rather than glowing nebulae, and mass-energy equivalence is a principle derived from Einstein's theory that relates mass to energy, but it is not directly linked to dark matter.