Dark energy is an unexplained force that is causing

The given blank can be filled with expansion of the universe to accelerate.

Explanation:

Dark energy is the term given to the force, which is considered to be the reason behind the expansion of the universe. It is a theoretical form of energy, which is believed to function against the gravity and covers the whole of the universe, and is responsible for the majority of the energy present in it and is making its expansion to accelerate.  

Dark energy was discovered by the astronomers in the 1990s and is the reason behind the expansion of the universe as it ages. It is found that this energy comprises approximately 70% of all the energy and matter in the universe.  

carbon atoms, hydrogen, oxygen, and nitrogen.