Dark Energy/Dark Matter

One extremely curious and slightly discouraging component of our knowledge of the universe is the existence of what is call Dark Matter and Dark Energy. Though we can't see or detect traces of either, leading theories suggest the universe is actually made more of these two than of regular matter. Humans, Earth, the Sun, our solar system, the Milky Way, and everything else is made up of elements, atoms, and cells that can be summarized as matter. All of this matter is thought to only make up about 5% of the universes total matter. The rest consists of about 25% Dark Matter and around 70% Dark Energy.

The only reason these two items are theorized is due to the apparent structure of the universe. Scientists have a hard time believing that the content of the universe-the stars, the galaxies-are all loosely bound by gravity from normal matter because it isn't strong enough. Without a structure, the stars might travel aimlessly through the universe, never clumping together to form any galaxies. So that is where scientists think something invisible that doesn't emit or reflect light comes into play and gives shape to the universe. 

Take a look at this video that illustrates and explains Dark Energy and Dark Matter in depth. Watch from beginning to 1:18 to learn the basics. Continue watching for additional information