As we seek to understand more about the Universe around us we continue to come across results that seem to create more confusion than clarity.
One specific example of this is the expansion of the Universe. The Universe is filled with mass, and mass will attract other mass due to the force of gravity. For this reason, scientists believed that the expansion of the Universe must be slowing down.
The end result being that it would either approach a zero expansion asymptotically - in other words, never quite stop. Or perhaps it would, like a spring, reach some maximum size and then begin to collapse back in on itself.
At the end of the 20th century though, work by researchers studying galaxies and, separately, by supernova specialists using the Hubble Space Telescope, found that the expansion of the Universe is not slowing down, but is instead speeding up.
The cause of this increase is unknown, but it was found that addition of a cosmological constant to Einstein's field equations of general relativity could account for this motion. But since there seemed to be no known mechanism to justify the addition of such a constant the moniker Dark Energy to denote its absence from direct observation.
Given, however, that it has yet to be directly detected, the question arises: Is Dark Energy real, or are we simply getting something wrong in our current framework we use to explain the dynamics of the Universe?
Is Dark Energy Just a Cop Out?
This is certainly a fair question. After all, the evidence for Dark Energy was completely unexpected, and we, as yet, have been unable to ascertain what would cause such accelerating expansion.
The truth is, though, that this is not uncommon in science, particularly physics and astronomy. We are constantly being surprised by results, or finding anomalies where we expected none.
Another, related, example is that of Dark Matter. There seems to be a significant amount of mass mission from the Universe, especially as you look at the motion and interaction of galaxies. This mass, which seems to not interact electromagnetically - that is, light does not interact with it - does not have a currently known associated particle or other entity.
So, naturally, scientists refer to it as being dark because it can not be detected or measured using the usual means. But that does not mean that it is not real. In fact, there is mounting evidence that dark matter is real matter.
Evidence That Dark Energy is Real
There is some indirect evidence that Dark Energy is a real phenomenon. First of all, surveying galaxies throughout the cosmos reveals some irregularities with their distribution; not what you would expect if the expansion of the Universe were constant or slowing down.
But the presence of a constant accelerating force would explain their distribution. Naturally, there could be one explanation, but this realization came before we even measured that the Universe was accelerating. Later observations only gave this measurement more ammunition.
The Nobel Prize winning discovery came in the Hubble Space Telescope measurement of distant supernovae. These events were found to be in galaxies that were receding more slowly than they should have been. This indicates that the Universe was once expanding more slowly than it is today.
More accurate measurements taken since that 1998 discovery have only confirmed this theory. But we still don't know what the driving force behind the acceleration, only that direct measurement and characterization of it remains elusive; hence the moniker Dark Energy.
But Could it Still Be Something Else?
So the evidence is pretty convincing the the expansion of the Universe is accelerating. That really only leaves two options: either there is some as-yet-unknown force or mechanism that is driving this acceleration, or perhaps our understanding of the forces of gravity is wrong.
There have been attempts to modify gravity before, but they have, almost universally, focused on non-relativistic forms of gravity theory. A cohesive reformulation of Einstein's Relativity Theories that could account for the observed acceleration, without the need of Dark Energy, remains elusive. But it is certainly possible.
In the end it seems a pretty safe bet that something is missing from out current understanding of the Universe, and neither option is very satisfying. Either we have completely missed some major driving force in the Universe, or one of the foundational theories of physics - one that gets tested and confirmed every nanosecond of every day - is somehow incomplete or wrong.