How much of the universe is dark matter?

This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters. The blend of blue and green in the center of the image reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found.
This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters. The blend of blue and green in the center of the image reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found. (Image credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University))

Astronomers estimate that roughly 85% of all the matter in the universe is dark matter, meaning only 15% of all matter is normal matter. Accounting for dark energy, the name astronomers give to the accelerated expansion of the universe, dark matter makes up roughly 27% of all the mass energy in the cosmos, according to CERN (the European Organization for Nuclear Research).

Astronomers have a variety of tools to measure the total amount of matter in the universe and compare that to the amount of "normal" (also called "baryonic") matter. The simplest technique is to compare two measurements. 

The first measurement is the total amount of light emitted by a large structure, like a galaxy, which astronomers can use to infer that object's mass. The second measurement is the estimated amount of gravity needed to hold the large structure together. When astronomers compare these measurements on galaxies and clusters throughout the universe, they get the same result: There simply isn't enough normal, light-emitting matter to account for the amount of gravitational force needed to hold those objects together. 

Thus, there must be some form of matter that is not emitting light: dark matter. 

Related: What is dark matter?

Different galaxies have different proportions of dark matter to normal matter. Some galaxies contain almost no dark matter, while others are nearly devoid of normal matter. But measurement after measurement gives the same average result: Roughly 85% of the matter in the universe does not emit or interact with light. 

Not enough baryons

There are many other ways astronomers can validate this result. For example, a massive object, like a galaxy cluster, will warp space-time around it so much that it will bend the path of any light passing through — an effect called gravitational lensing. Astronomers can then compare the amount of mass that we see from light-emitting objects to the mass needed to account for the lensing, again proving that extra mass must be lurking somewhere.

Astronomers can also use computer simulations to look at the growth of large structures. Billions of years ago, our universe was much smaller than it is today. It took time for stars and galaxies to evolve, and if the universe had to rely on only normal, visible matter, then we would not see any galaxies today. Instead, the growth of galaxies required dark matter "pools" for the normal matter to collect in, according to a lecture by cosmologist Joel Primack.

Lastly, cosmologists can look back to when the cosmos was only a dozen minutes old, when the first protons and neutrons formed. Cosmologists can use our understanding of nuclear physics to estimate how much hydrogen and helium were produced in that epoch.

These calculations accurately predict the ratio of hydrogen to helium in the present-day universe. They also predict an absolute limit to the amount of baryonic matter in the cosmos, and those numbers agree with observations of present-day galaxies and clusters, according to astrophysicist Ned Wright.

Alternatives to dark matter

Alternatively, dark matter may be a misunderstanding of our theories of gravity, which are based on Newton's laws and Einstein's general relativity.

Astronomers can tweak those theories to provide explanations of dark matter in individual contexts, like the motions of stars within galaxies. But alternatives to gravity have not been able to explain all the observations of dark matter throughout the universe. 

All the evidence indicates that dark matter is some unknown kind of particle. It does not interact with light or with normal matter and makes itself known only through gravity. In fact, astronomers think there are trillions upon trillions of dark matter particles streaming through you right now. Scientists hope to nail down the identity of this mysterious component of the universe soon.

Originally published on LiveScience.

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Paul Sutter Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

  • rod
    Another report that raises questions for me like when and how did the BB create DM? Computer simulations are good, but I think must be carefully balanced. The wrong amount of DM appearing after the BB event, the universe does not evolve like what we see today, said to take about 13.8 billion years. This holds true for various aspects of the Standard Model, dark energy, and problems like the cosmological constant. While DM is said to be seen in galaxies (generally with low redshifts), that observation does not explain when DM appears in the expanding universe or how DM managed to form just right for the expansion to avoid a collapse of the expanding universe (too much DM). Perhaps more fine-tuning problems are here.

    I ask like questions in other reports too about BB cosmology.
  • rod
    What is dark energy? | Forums

    Another interesting report with posts asking questions. DE could be a fine-tuning problem as well in the BB model. Somehow, nature popped up all right and ready, so the expanding universe did not destroy itself at the start :)
  • DrRaviSharma
    DM is everywhere including where there is no visible or EM sensible matter. But as I am writing a paper on how DM creates Matter-energy in the form of nucleus and Atoms, if at intermediate to that nucleus forming stage, if we measure gravity, we will find some even where there is no matter so that is why we are able to feel its gravitational effects.
    (Dr. Ravi Sharma, Ph.D. USA)
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