How do we know dark energy is out there?

As a a matter of fact, as Discovery News' Nicole Gugliucci explains, some astronomers do not believe dark matter exists.

Before 1998, when two competing teams of researchers co-announced their findings that the universe is expanding at an accelerating rate, scientists were certain that the growth of the universe had to be slowing down. This discovery forced scientists to come up with an explanation for the force that could be working against gravity.

Dark energy was originally predicted as a cosmological constant throughout our universe by Albert Einstein. In May of this year, scientists collecting data for four years on nearly a quarter of a million galaxies stretching back over seven billion years concluded that dark energy is out there, as predicted by Einstein.