An overview of dark energy.

Every object humans have ever studied, such as stars, planets (including the Earth), asteroids, gas clouds and living creatures such as ourselves, is composed of ordinary matter, which is made up of atoms. Interestingly, however, ordinary matter only composes 5% of the mass of the Universe. The remaining 95% is made up of dark matter and dark energy. This post is about dark energy which, as shown in the diagram below, constitutes about 70% of the total mass of the Universe.

Dark energy now

This article is the third in my series on cosmology, which is the study of the origin and evolution of the Universe as a whole.

The complete list of cosmology posts is at 

Evidence for Dark Energy

As discussed in my previous post  Universe past present and future, astronomers have known since the 1920’s that the Universe is expanding. By this we mean that the distance between galaxies increases over the course of time, although the individual galaxies don’t get any bigger.

Universe Expanding

Up until 1998 the generally held view was that the expansion of the Universe was slowing down. There was a good reason for believing this.  When astronomers applied Einstein’s theory of general relativity to the Universe, it predicted that the gravity caused by all the matter in the Universe would slow down the expansion. This is shown in the graph below.

This diagram shows the expansion of the universe has slowed down over time.  Along the x-axis is time and the y-axis shows the distance between galaxies. After the initial big bang the Universe expands very rapidly. As time progresses the Universe continues to expand, but at a slower and slower rate, so the distance between galaxies increases more gradually.

Einstein’s theory predicted that the more matter in the Universe, the greater the slowing in the rate of expansion. Before 1998, cosmologists believed that if there was enough matter in the Universe the expansion would eventually halt and it would actually start to contract.  This would mean that the galaxies would eventually start getting closer together again. As time progressed the rate of contraction would get faster and faster, and the eventually the entire Universe would collapse in the so-called “big crunch”.

This diagram shows the “big crunch”.  Before 1998 there was no consensus among cosmologists about whether a big crunch would occur; we just didn’t have accurate enough measurements of the amount of matter in the Universe.

In 1998, following years of observations to accurately measure both the distances  of far away galaxies and how fast they were moving away from us, it was discovered that the expansion of the Universe was actually speeding up.


The 1998 results came a great surprise to most astronomers and was of such huge importance that Science magazine named it scientific breakthrough of the year for 1998.  Three of the discovers later won the 2011 Nobel prize for physics.

Because,  Einstein’s theory of general relativity predicts that gravity will slow down the Universe’s expansion, there must something acting to counteract this. This is dark energy.

How much dark energy is there?

Our current theories are that dark energy is completely evenly distributed throughout the Universe. Unlike ordinary matter it doesn’t clump together – there are no objects made out of dark energy.

Cosmologists have worked out that the actual amount of dark energy needed to counteract gravity and cause the observed speeding up in the rate of expansion of the Universe is actually very small. It is usually expressed as a mass equivalent  and works out at about 0. 0069 trillionths of a gramme per cubic kilometer of space.  This is an almost unfathomably tiny amount – 150 billion times times smaller than a small grain of sand. However, most of the Universe is empty space and although ordinary matter is found at high densities in objects such as stars and planets, the spaces between these objects are so great that averaged over the entire Universe the density of ordinary matter works out at the incredibly low figure of about 0.0005 trillionths of a gramme per cubic kilometer of space – 2 trillion times smaller than a grain of sand. Dark matter has a density of roughly 0.0027 trillionths of a gramme per cubic kilometer of space  averaged out throughout the Universe.

What form does dark energy take ?

Cosmologists aren’t in agreement about what dark energy is. However, the simplest explanation of dark energy is something called the cosmological constant,  which physicist usually give the symbol Λ (lambda) first proposed by Albert Einstein  in 1917.

Albert Einstein

Albert Einstein

The cosmological constant can be considered as the ‘vacuum energy’. The idea being that empty space isn’t truly empty but contains a small amount of residual energy.   It is this small amount of residual energy which causes empty space to gradually expand.

Another point about the cosmological constant is that, because it is a constant, it has the same value throughout the entire Universe and doesn’t vary with time.

When Einstein applied his newly discovered theory of general relativity to the Universe it predicted that gravity would, over time, cause the objects in it to get closer and closer together,  causing the whole Universe to collapse. Einstein, like most physicists at the time, believed the Universe must be static-  neither contracting nor expanding. To make general relativity predict a static Universe, he had to introduce the cosmological constant to counteract the force of gravity.

After it was discovered in the late 1920s that the Universe was expanding, the cosmological constant was no longer needed to force a static Universe. Einstein, who didn’t like the idea of empty space having some energy, discarded it and said that introducing it was the “greatest blunder of my life”.  For the next 70 years most cosmologists believed that the cosmological constant was zero.  Interestingly, since 1998 many cosmologists have come to believe that the cosmological constant may be non-zero. Perhaps Einstein didn’t make a blunder after all!


The cosmological constant is not the only explanation of dark energy and is by no means accepted by all cosmologists.  There are other more exotic explanations called scalar fields in which the average dark energy density does change over time. Perhaps in the next 20 years we will determine the nature of dark energy.

31 thoughts on “An overview of dark energy.”

  1. […] Along with these 4 forces there are different basic parameters such because the  mass of the proton and electron and the typical densities of bizarre matter, darkish matter and darkish power within the Universe. (See my earlier posts for extra info on dark matter and dark energy.) […]


  2. […] In addition to these four forces there are other fundamental parameters such as the  mass of the proton and electron and the average densities of ordinary matter, dark matter and dark energy in the Universe. (See my previous posts for more information on dark matter and dark energy.) […]


  3. Dark energy is the illusion that light escapes towards itself therefore generating echoes of gravity away from a gravitational force.
    When light or material objects “gravitate” in the emptiness of space, antigravitational illusions deform the appearance of the visible spectrum of space like ripples of water distorting the depths of a crystal clear lake.
    The inferior force of our knowen gravity is a spectrum of tiny holes within the never ending Lens of the universe.
    Dark energy coagulates in to Dark Matter forming black holes when gravity is still.
    Infinite energy or propulsion is achievable when isotopes like Protium, Deuterium or Tritium are thrown towards gravity itself.


  4. Any layman can discover Einstein’s Special Relativity all on their own. It’s hard to believe, but it’s true. The physicists just make it look complicated by not describing it in its entirety, but by presenting it in fragments that are only linked by equations.


    These 9 mini videos are to show the layman what is behind the mysteriousness of Special Relativity. The point is that even the “Non-Scientist” can figure this stuff out.


  5. Thanks a lot for your like of my page, and also for making me discover the concept of “dark energy”! It looks like I’ll have to look around your page a bit to learn more about the scientific aspects of life and the universe ;).


  6. I love your blog. Thank you for creating it.

    I’m a layman at best when talking about physics and cosmology but like to do so. As a kid in the late1940s, I came up with a simplistic answer as to how big space is. I’m assuming that lots of folks have thought of the same thing because it was so obvious to me. I reasoned that space has no limits because if one were able to travel at infinite speed in a straight line and able to travel through matter, there would be nothing to hinder one’s going on forever because if one encountered some type of obstruction the obstruction would be infinitely thick or there would be more “empty” space on its back side.

    Also I’m presently wondering if the “empty” space between an atom’s electrons and its nucleus is the same type of “empty” space between planets and galaxies. It seems to me it would be.


  7. I LOVE these topics and want MORE. Question: How does the new discovery of the UNCONSTANT speed of light affect this “cosmological constant”?

    Is ANYTHING “constant,” really? Buddhists say NOT. I agree.

    Best to you,



    1. Hi Sally,

      Thanks for your comment and I’m glad you’re enjoying my blog.

      If you want to view any of my other posts on cosmology then please click on the “Cosmology” tag in the “Tag Cloud” to the left of this post or the Cosmology Category at the bottom of the post. (This may not work if you’re reading this on a small screen mobile device 🙂 )
      I know there have been experiments which have suggested that the speed of light may not be constant, one of which is described in the lnk below

      However, my understanding is that these result have not yet been generally accepted and confirmed as being correct. If this proves to be true then the implications could be quite profound !

      The Science Geek

      Liked by 1 person

  8. I was very interested in the comments about expansion -‘expansion into what?’ I think the point is that the expanding balloon concept does not work. Space and time are interlinked, and the latter is infinite in the future (QED) so there is no real contradiction.


    1. Thanks for your comment. There are number of theories about the ultimate fate of the Universe. The scenario which convinces me most is where the Universe continues to expand and cool forever and eventually contains dark energy and nothing else. This scenario is sometimes called the big freeze and I’m planning to write a post about it.

      The Science Geek


  9. I should just elaborate: as a physicist, Einstein took a long time to master the mathematics of General Relativity. In the intervening years, the mathematicians have taken over the field (in fact, Ed Witten, the foremost theorist of our era, has won more significant prizes in Mathematics than in physics).

    That by way of introduction: the reason Einstein called the cosmological constant his biggest blunder was because he objected to the introduction of terms in a model that do not have a physical explanation. If the cosmological constant exists, it must have a cause. That means that space is filled up with something, and that something establishes a preferred reference frame. In other words, the cosmological constant undermines the assumptions of special and general relativity, which both hold that there is no preferred reference frame.

    The mathematical physicists that have taken over the field don’t worry too much about this, because they are engaged in curve fitting. As long as there are enough parameters in the theory to fit the data, they are happy. If they can’t fit the data, they just add more parameters.

    I believe that our understanding of dark energy is at the level of caloric in the early days of thermodynamics, before the kinetic theory of gases was developed. I believe that understanding its nature will require a complete revolution in our theories of reality.

    You can see more critical analysis of modern particle theory at:


    1. Thank you very much for making your interesting and thought provoking points. Clearly there are other explanations of dark energy other than a non zero cosmological constant, such as quintessence. However, at the moment a small positive cosmological constant appears to be the most widely accepted explanation among cosmologists and is probably the simplest. This is why I decided to focus on it in this post. I agree with your point that it may not turn out to be the correct explanations.


  10. I’m really impressed with the simple and didactic way you explain intricate matters!
    I would like to also have this ability! I work with non-equilibrium statistical mechanics. And I do not see a clear and simple way to explain what is important in my area of research. Congratulations on your post!


    1. Hi Tania,

      Thanks for your comment. It is always difficult when writing a popular science blog to get the balance right between getting the information across, so that the blog is informative, yet at the same time not making the blog too technical, and thus too hard to understand for a non scientist.

      Hopefully I’ve hit the spot

      The Science Geek


  11. Very interesting article which, as all good articles should, raises some observations and questions. My first observation is how, by quite reasonably using terms such as “ordinary matter”, “dark energy” and “dark matter” the human mind immediately starts with preconceptions coloured by the words “dark”, “ordinary” etc, perhaps we could have used the words “fragile energy” ,”discrete matter” or “hidden energy” and “concealed matter” – all of which terms would have brought their own preconceptions according to how we humans use language. My point is that we can’t help describing these incredible concepts in human terms. So I try and divorce the concepts from the words, to try and get at the essence of what is being described, and admittedly don’t get very far, but sometimes it does help – as Shakespeare said “A rose by any other name would smell as sweet”
    My other observation/question is: If the universe is expanding, and space between galaxies is getting bigger, what is it that the universe is expanding in? Is there a huge super-universal space that is accommodating all the ordinary/dark matter, galaxies, black holes etc that we regard as the “universe” – or is the universe its own space? Are there boundaries to the “space” within which we think the universe is expanding? or is there no “other” space than that which is currently expanding? – This is getting hard to think about!


    1. Thank you Graham for your interesting comments. To me the name dark energy is perfect because it isboth dark in the sense that it is invisible, other than its effect on speeding up the expansion of the Universe and also dark in the sense of mysterious/ unknown because we still don’t know what it is. Although it may be due to a small cosmological constant, which as stated in the post was introduced by Einstein in 1917, but later disgarded.
      With regard to the expansion of the Universe, it is space itself that gets bigger, which I know can be a tricky concept to get your head around.

      The Science Geek


  12. I find this to be awesome, “As discussed in my previous post titled Universe past present and future, astronomers have known since the 1920’s that the Universe is expanding. By this we mean that the distance between galaxies increases over the course of time, although the individual galaxies don’t get any bigger.”


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