Dark Energy:

Dark energy is similar to dark matter. While matter slows things down by pulling it together, energy pushes things away. Depending on the source, anywhere from 65%-75% of the universe is made of dark energy. This explains why our universe is constantly expanding and how galaxies are accelerating away from one another. Also, like dark matter, scientists are sill unable to explain what dark energy is.

 

The leading theory at the moment is that, like Eienstein suggested, dark energy is just a property of space itself. It's likely that, like the gravitational constant, dark energy is also a cosmological constant. Until all other theories are explored, this just remains as the best of many theories. One of the best theories, although probably wrong, is that dark energy is a new fluid-like mass unknown to man. Like matter has a gravitational pull, this theory gives dark energy a negative gravitational pull. To learn more about dark energy and what we don't really know, check here.