By observing the universe, scientists have learned that galaxies are moving farther away. They believe that the universe has been expanding, like a balloon being blown up, since the big bang. This is phenomena is known as dark energy.
Dark matter, on the other hand, is the mass in the universe that cannot be accounted for by matter that we can see. While dark energy works to expand the universe, dark matter tries to hold it together. Both cannot be seen by light, nor do they seem to be affected by normal matter. The craziest part? Dark energy and matter account for most of the stuff in the universe.