DARK ENERGY

Dark energy is even more mysterious, and its discovery in the 1990's was a complete shock to scientists. Previously, physicists had assumed that the attractive force of gravity would slow down the expansion of the universe over time. But when two independent teams tried to measure the rate of deceleration, they found that the expansion was actually speeding up. One scientist likened the finding to throwing a set of keys up in the air expecting them to fall back down-only to see them fly straight up toward the ceiling.

Scientists now think that the accelerated expansion of the universe is driven by a kind of repulsive force generated by quantum fluctuations in otherwise "empty" space. What's more, the force seems to be growing stronger as the universe expands. For lack of a better name, scientists call this mysterious force dark energy.

Unlike for dark matter, scientists have no plausible explanation for dark energy. According to one idea, dark energy is a fifth and previously unknown type of fundamental force called quintessence, which fills the universe like a fluid.

Many scientists have also pointed out that the known properties of dark energy are consistent with a cosmological constant, a mathematical Band-Aid that Albert Einstein added to his theory of general relativity to make his equations fit with the notion of a static universe. According to Einstein, the constant would be a repulsive force that counteracts gravity, keeping the universe from collapsing in on itself. Einstein later discarded the idea when astronomical observations revealed that the universe was expanding, calling the cosmological constant his "biggest blunder."

Now that we see the expansion of the universe is accelerating, adding in dark energy as a cosmological constant could neatly explain how space-time is being stretched apart. But that explanation still leaves scientists clueless as to why the strange force exists in the first place.