The Universe is a mysterious place that we have only begun to study and it seems that every time scientists answer one cosmological question, a few more arise, making the complete understanding of the space we live in a distant possibility.
Dark energy is one of the mysteries, as it is not only elusive, being theorized but never actually observed, but also a concept whose understanding seems closer with every new find yet never fully in our grasp. 'What exactly is this dark energy?' 'What does it do?' 'How does form?' are just some questions that astrophysicists are trying to answer but never seem to agree on.
Modern views of this exotic form of energy present it as one that can permeate all the space and most likely caused an increase in the expansion rate of the Universe. At least that's what recent observations of the expansion seem to indicate.
For now, two forms of this energy have been theorized, one being a cosmological constant, that apparently fills space homogeneously, and the other being represented
by scalar fields having an energy density that varies both in space and in time.
One of the problems regarding this dark energy is the fact that, though it is believed to have a negative pressure, drawing in matter from its surroundings, it actually causes an expansion of the entire Universe. The idea of negative pressure is not something we can easily understand. While every substance has a positive pressure, like gases, liquids and even light, which push outward on their surroundings, dark energy pulls on its surroundings, like a solid stretches to support a hanging weight.
It has been calculated that this negative pressure has a gravitational repulsive effect even greater that the gravitational attraction caused by the energy itself, thus overwhelming all other forms of gravitational attraction, which in turn produces the accelerating expansion of the universe.
But how can pulling be causing a pushing effect? Well, both types of pressure being non-gravitational forces, they cause the movement of substances within space but never really change space itself. On the other hand, they cause a gravitational attraction or repulsion that acts on the fabric of space, decreasing and increasing the amount of space between substances, respectively.
Since they act in opposite directions, this is what actually produced the current size of the Universe, so scientists say this energy must exist in order to reconcile the current measurements of the expanse of space with the calculated amount of matter in the Universe.
But experiments have yet to observe this energy in action, since its small density makes it really hard to detect in laboratory. To give you an idea of this density, a space of 100 billion cubic kilometers made entirely of dark energy weighs just one thousandth of a gram. For comparison, the volume of the Earth is of a little more than 100 billion cubic kilometers.
Where exactly can dark energy be found? Everywhere, yet nowhere. Some recent theories seem to have come up with interesting locations and probably the most intriguing one says that they cannot be found in our three-dimensional space.