Dark energy is a mystery concept that makes up 68% of our universe and is causing it to expand rapidly.

    Einstein had realized that empty space isn't empty -- it has its own energy, therefore is not empty. He also came up with other theories that tested temporary particles, but was off by such a large amount that it's almost sad how wrong he was. He continued to research this and recommended that a new theory of gravity would need more data to work with.

    Dark matter also plays a role in dark energy. Dark matter isn't visible, but we know for a fact that it's not antimatter or baryons (dark clouds of matter). Overall, dark matter and dark energy are thought to be responsible for the expansion of the universe, although this hasn't been proven. Current research is still being done. The Fermi Mission expanded their search for dark matter on August 12th, 2016, and more research has been done since.