70% of the mass of the universe is made of a mysterious stuff we call Dark Energy. It cannot be seen directly, but its existence is inferred through the way it affects the motion of galaxies. We have a limited understanding of what this dark stuff is supposed to be. The best explanation comes from quantum mechanics. If you take a box of empty space, vacuum, and if you were able to weigh it with great accuracy, you would find out that it ain't really empty... Quantum mechanics allows for a level of controlled fuzz at the microscopic level: vacuum is really not empty, but looks more like a soup of particles being constantly created and destroyed out of nothingness. The more massive a particle that is created from nothingness, the shorter it lives. The end result is a picture of the vacuum which is a breathing living animal of random particles... hence, the empty box will have some weight. The most amazing thing about this picture is that it implies that the vacuum of space "anti-gravitates": regular matter collapses under the pull of gravity; vacuum mass - let's call it dark energy - does the opposite! It creates a repulsive gravitational effect. The end result is that empty space will tend to expand violently because of dark energy. So, there's a competition in the universe between a pull from the regular matter in it, and the repulsion from the dark energy in the vacuum. Currently, dark energy is winning out and we see the universe is undergoing an expansion: the fabric of space is being stretched to larger and larger sizes! Now, here's the depressing part. The story I just outlined is only a partial explanation. There are other layers to understanding dark energy as a condensate of stuff in the vacuum. However, if we were to take this most simple explanation seriously, we can try to estimate the amount of dark energy expected and compare it with the amount we actually measure... we find that a cube of empty space - with a size of 1 centimeter for its sides - weighs 10 to the power of minus 32 kilograms... That's tiny, by measurable on astrophysical scales! The best sensible prediction we have from quantum mechanics gives a value slightly bigger: 10 to the power 112 times bigger... it is then safe to say experiment doesn't agree with theory... And we are still to understand what 70% of the universe if made of!