Gravity is attractive, a fact that nearly every school age child would be able to tell you. That is why we are stuck to the surface of the Earth, why the Moon orbits around us and what keeps our tiny planet bound to the Sun.
Naturally then, when we look out and see that the Universe is filled with mass we draw the inevitable conclusion that, while the Universe may be expanding, it has to be slowing down. After all, with all that mass pulling all the other mass inwards the Universe should, given enough time eventually slow to a stop or perhaps collapse back in on itself.
This is what we expected to see anyway. But sometimes the truth is unexpected, and even a bit strange.
Often overlooked, the first real evidence that perhaps things were not as expected came in the early 1990s when astronomers were looking at the distribution of galaxies throughout the Universe.
The distribution of galaxies was found to be irregular, at least in terms of how they matched with theoretical predictions. Two years later, in 1992, another paper was published that found that these irregularities could be explained if one added a Cosmological Constant to the Einstein Field equations of general relativity.
This realization wasn't proof of anything necessarily, but provided a convenient parameter to fit an otherwise strange data set.
Hubble's Survey of Distant Supernovae
Once the Hubble Space Telescope was launched in 1990 it began scanning the skies, beaming back brilliant pictures of the cosmos. As time went on, the telescope began to peer deeper and deeper into the Universe, effectively looking back in time.
In 1998 a survey was published of distant supernovae. Type Ia supernova are used as a "standard candle" of astronomy because they always occur at the same white dwarf mass, which means that they should all have the same intrinsic brightness.
By measuring how bright the supernovae appear to us, we can estimate the distance to these objects using the inverse square law. We can also use the measured redshift of the events to measure how quickly they are receding from us.
What the survey found was that billions of years ago the Universe was expanding more slowly than it is today. In other words, the expansion of the Universe is accelerating.
This revelation did not fit into the normal framework of general relativity, except when a Cosmological Constant was added to the Einstein Field equations. We currently don't have an explanation for where this constant would come from and since we can't seem to "see" it or measure it, we have dubbed its source Dark Energy.
Those responsible for this work were awarded the Nobel Prize in Physics in 2011 for their efforts.
Other Evidence of Dark Energy
While the physics behind the methods are somewhat complex, researchers were essentially able to gain more accurate distance and recessional velocity measurements (out to about 7 billion light-years away) and found that the expansion of the Universe was indeed accelerating, and doing so in a relatively uniform manner.
How Much Dark Energy is Out There?
Another study that was done was the measurement of the Cosmic Microwave Background (CMB). This measurement gives scientists a measurement of the background temperature (or energy) of the Universe. As the cosmos expands, it also cools. This is one way that we can measure the relative age of the Universe.
One thing that was deduced from measurements of the background microwave radiation was that the Universe is nearly flat; that is, nearly without curvature. The only way, as far as we can tell, for the Universe to be flat is if it has a certain mass/energy density.
But adding up all of the ordinary, luminous matter, gas and Dark Matter only gets us about 27% of the way there. That means that about 73% of the Universe would have to be made up of Dark Energy to account for the short fall.
What is Dark Energy?
This all begs the question: What is Dark Energy? The answer is that we are not sure. It could be some mysterious energy field that we have yet to find and measure. Or perhaps it is nothing at all, maybe it is relativity theory that is incorrect. The only thing that is known for sure is that more time and data are needed to investigate the underlying truth of Dark Energy.