Dark Energy. Astronomers say roughly 68% of the universe is made up of something called dark energy. But what is it? There are a few possibilities. 

A Property of Space

     Albert Einstein realized that empty space isn't just nothing. His initial theories of gravity contained a cosmological constant. He theorized that empty space has its own energy. Because this energy is a property of space it would not be diluted as space expands, but even more energy will be produced, causing the universe to expand faster and faster.



Or is it a new dynamic fluid.

     One possibility is that dark energy is some sort of dynamic fluid or field that has never been discovered before. But that is a theory of which little else is known, like what is actually is or what it interacts with.

Could Einstein be wrong?

     There is a third possibility that Einstein's theory of gravity is not correct. If that's the case we don't know what kind of theory it would be but any new theory would change how galaxies and clusters behave, changing the way we view the universe. If the cosmological constant is not a true constant there are a few theories as to the outcome of the universe........ 


Source:

"Dark Energy, Dark Matter." NASA Science. 5 Jun 15. NASA. Web 7 Apr 16.
<http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/>