Dark energy makes up roughly 70 percent of the universe, yet we know nothing about it.

Around 25 percent of the universe is the equally mysterious dark matter, leaving just five percent for everything that we can see and touch -- matter made up of atoms.

Dark energy is the placeholder name scientists have given to the unknown force causing the universe to expand faster and faster over time.

But some recent cosmic clues have been chipping away at the leading theory for this phenomenon, which could eventually mean humanity will have to rethink our understanding of the universe.

And with several new telescopes taking aim at the problem, scientists hope to have some concrete answers soon.

Here is what you need to know about what many scientists have called the greatest mystery in the universe.

No one knows. It is invisible and it does not interact with matter or light.

And it may not even exist.

This story begins -- like everything else -- at the Big Bang around 13.8 billion years ago, when the universe first started expanding.

Since then, there has been "cosmic tug-of-war" between two mysterious........

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