What are dark matter and dark energy and how do they affect the structure and evolution of the cosmos?
Dark matter is a form of matter that does not emit, absorb, or reflect light, which makes it invisible to telescopes.  Despite this, its existence is inferred from its gravitational effects matter that is visible.

Dark energy is an enigmatic form of energy that permeates all of space and is responsible for the observed acceleration of the expansion of the universe.

Together, dark matter and dark energy profoundly impact the structure and evolution of the universe.  Dark matter provides the gravitational force needed for galaxy formation and helps maintain their structure.  Dark energy counteracts the attractive force of gravity, causing the universe to expand at an accelerating rate. 

The interplay between dark matter and dark energy determines the fate of the universe.  If dark energy continues to dominate, the universe will keep expanding faster and faster, eventually leading to a scenario known as the "Big Rip."