What is Dark Energy?

Up until the 1990’s it was generally believed the expansion rate of the Universe since the Big Bang was slowing down. The Universe is full of matter with gravitational forces that would attract each other and eventually collapse. Well, that’s what everyone thought; that was what theoretically should happen.

In 1998, Hubble Space Telescope observed some very distant supernovae that occurred some billions of years ago that showed that at that time the Universe was expanding very slowly compared to today.

Image courtesy: NASA.gov

Shocking!!! This broke all formerly held theories. How could it be possible? What was happening? No one knew how. But they came to a conclusion that something was causing it. What could it be?

Scientists came up with three explanations:

1. Maybe this was a result of discarded version of Einstein’s theory that contained what was called “cosmological constant”.

The first explanation is that it is a property of space. Einstein was the first person to realize that empty space is not empty at all. Space has amazing properties.

Einstein discovered one property which is that it is possible for more space to come into existence.

The version of Einstein’s theory that contains a cosmological constant predicts that “empty space” can possess its own energy.

Because this energy is space’s own property it will not dilute as the Universe expands. As more space comes into existence more of this energy would appear. As a result this energy would cause the Universe to expand faster and faster.

Still it is a mystery on why this energy should exist and why it will have right amount of energy to cause the observed acceleration of the Universe’s expansion.

2. Maybe there was a strange energy fluid that filled space.

The second theory/explanation is the presence of the fifth element called the “quintessence,” after the fifth element of the Greek philosophers. However, if such an element exists the mystery on what is like, what it contains, and other “what”, “why”, and “how” questions remain unanswered.

3. The third explanation was a doubt if there was an error in Einstein’s theory of gravity.
If so, this would mean that whatever theories that currently exist for normal matter in galaxies and galaxy clusters, and other existing theories would have to be explained with an entirely new theory.

So the mystery continues. It is an important mystery because it turns out that the Universe is made of of roughly 68% Dark Energy.

Dark Matter is 27% and the rest, the normal observable matter, is about 5% !!!

What is Dark Matter?

The visible universe—including Earth, the sun, other stars, and galaxies—is made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter

Scientists are much more certain what dark matter is not than what it is.

  • First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 27% required by the observations.
  • Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. This is known because baryonic clouds can be detected by their absorption of radiation passing through them.
  • Third, dark matter is not antimatter, because the unique gamma rays that are produced when antimatter annihilates with matter are not seen.
  • Finally, large galaxy-sized black holes are ruled out on the basis of how many gravitational lenses that can been seen. High concentrations of matter bend light passing near them from objects further away; enough lensing events are not seen to suggest that such objects to make up the required 25% dark matter contribution.

At this point, there are still a few dark matter possibilities.

1. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as Massive Compact Halo Objects, or “MACHOs”.
2. Another most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).

Despite this, Dark Matter, along with Dark Energy, still continues to be a mystery and lots more data is required to understand this.


This Hubble Space Telescope composite image shows a ghostly “ring” of dark matter in the galaxy cluster Cl 0024+17.
Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)
Image and information courtesy:space.com


Image courtesy: Mysterious Universe: Dark Matter vs Dark Energy.
Information source credit: NASA

Recommended reading for children – Mysterious Universe: Dark Matter vs Dark Energy