Even under the presence of gravity, the universe is expanding. Is gravity so weak to hold the universe? Or there is something else, in the dark? Dark energy. Looking back into matter in the universe, the universe consists of 5% normal matter, 27% dark matter and 68% dark energy. What is this dark energy? Why is it in so large proportions? Dark energy is the factor behind the expansion of the universe. Back in late 19th century, astronomer Edwin Hubble discovered that the universe was expanding. Then scientists believe that universe will stop expanding after some time due to gravity. But later, in 1990 Hubble Space Telescope discovered that the expansion was faster than earlier, it was accelerating. And this was due to presence of something mysterious in the space, they called it dark energy. There are many possibilities of what dark energy could be. One explanation for dark energy is that it is a property of space. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Second explanation could be, empty space can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of the energy of space would appear. As a result, this form of energy would cause the universe to expand faster and faster. However this expansion can only be observed between distant galaxies and clusters, since distance between stars and planets is small-hand gravity is stronger. Another explanation of dark energy can be understood through quantum theory of matter, but that's for later. Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the universe, but it would also affect the way that normal matter and galaxies and clusters of galaxies behaved. This could change everything thus we need mute theories, ideas and that is to completely understand our universe. For now it's still a mystery.