Is There Proof of Dark Matter?

Updated on April 6, 2018
Sam Brind profile image

Sam Brind holds a master's in physics with theoretical physics (MPhys) from the University of Manchester.

Introduction to Dark Matter

The current standard model of cosmology indicates the mass-energy balance of our universe to be:

  • 4.9% - 'normal' matter
  • 26.8% - dark matter
  • 68.3% - dark energy

Therefore, dark matter makes up almost 85% of the total matter in the universe. However, physicists currently don't understand what dark energy or dark matter is. We do know that dark matter interacts with objects gravitationally because we have detected it by seeing its gravitational effects on other celestial objects. Dark matter is invisible to direct observation because it doesn't emit radiation, hence the name 'dark'.

M101, an example of a spiral galaxy. Notice the spiral arms extending from a dense centre.
M101, an example of a spiral galaxy. Notice the spiral arms extending from a dense centre. | Source

Radio Observations

The main piece of evidence for dark matter comes from the observation of spiral galaxies using radio astronomy. Radio astronomy uses large collecting telescopes to collect radio frequency emissions from space. This data will then be analysed to show evidence for extra matter which can't be accounted for from observed luminous matter.

The most commonly used signal is the hydrogen 21-cm line. Neutral hydrogen (HI) emits a photon of wavelength equal to 21 cm when the atomic electron's spin flips from up to down. This difference in spin states is a small energy difference, and hence this process is rare. However, hydrogen is the most abundant element in the universe, and hence the line is easily observed from the gas within large objects, such as galaxies.

An example spectra obtained from a radio telescope pointed at the M31 galaxy, using the 21cm hydrogen line. The left image is uncalibrated and the right image is after calibration and removal of the background noise and the local hydrogen line.
An example spectra obtained from a radio telescope pointed at the M31 galaxy, using the 21cm hydrogen line. The left image is uncalibrated and the right image is after calibration and removal of the background noise and the local hydrogen line.

A telescope can only take an observation of a certain angular segment of the galaxy. By taking multiple observations that span the whole galaxy, the distribution of HI in the galaxy can be determined. This leads, after analysis, to the total HI mass in the galaxy and hence an estimate of the total radiating mass within the galaxy, i.e. the mass that can be observed from emitted radiation. This distribution can also be used to determine the velocity of the HI gas and hence the velocity of the galaxy throughout the observed region.

A contour plot of the HI density within the M31 galaxy.
A contour plot of the HI density within the M31 galaxy.

The velocity of the gas at the edge of the galaxy can be used to give a value for the dynamic mass, i.e. the amount of mass causing the rotation. By equating the centripetal force and gravitational force, we obtain a simple expression for the dynamic mass, M, causing a rotation velocity, v, at a distance, r.

Expressions for the centripetal and gravitational forces, where G is Newton's gravitational constant.
Expressions for the centripetal and gravitational forces, where G is Newton's gravitational constant.

When these calculations are performed the dynamic mass is found to be an order of magnitude larger than than the radiating mass. Typically, the radiating mass will only be about 10% or less of the dynamic mass. The large quantity of 'missing mass' that isn't observed through radiation emission is what physicists call dark matter.

Rotation Curves

Another common way of demonstrating this 'fingerprint' of dark matter is to plot the rotation curves of galaxies. A rotation curve is simply a plot of the orbital velocity of gas clouds against the distance from the galactic centre. With only 'normal' matter, we would expect a keplerian decline (rotation speed decreasing with distance). This is analogous to the speeds of planets orbiting our sun e.g. a year on Earth is longer than on Venus but shorter than on Mars.

A sketch of rotation curves for observed galaxies (blue) and the expectation for keplerian motion (red). The initial linear rise shows a solid body rotation in the centre of the galaxy.
A sketch of rotation curves for observed galaxies (blue) and the expectation for keplerian motion (red). The initial linear rise shows a solid body rotation in the centre of the galaxy.

However, the observed data doesn't show the keplerian decline that was expected. Instead of a decline, the curve stays relatively flat up to large distances. This means that the galaxy is rotating at a constant rate independent of the distance away from the galactic centre. To maintain this constant rotation speed the mass must be linearly increasing with radius. This is the opposite of observations that clearly show galaxies that have dense centres and less mass as distance increases. Hence, the same conclusion as earlier is reached, there is additional mass within the galaxy that is emitting no radiation and hence hasn't been directly detected.

The Search for Dark Matter

The problem of dark matter is an area of current research in cosmology and particle physics. Dark matter particles would have to be something outside of the current standard model of particle physics, with the leading candidate being WIMPs (weakly interacting massive particles). The search for dark matter particles is very tricky but potentially achievable through either direct or indirect detection. Direct detection involves looking for the effect of dark matter particles, passing through the Earth, on nuclei and indirect detection involves searching for potential decay products of a dark matter particle. The new particles may even be discovered in high energy collider searches, such as the LHC. However it is found, the discovery of what dark matter is made out of will be a huge step forward in our understanding of the universe.

Questions & Answers

    © 2017 Sam Brind

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      • savvydating profile image

        Yves 

        3 months ago

        Hello Sam....Your explanation about "missing mass" is probably the only thing I actually understand right now. Lol. Nevertheless, I appreciate your manner of writing about science/physics in a way that is at least somewhat understandable to a novice such as myself.

        Interesting article and....profile pic. ;)

      • Sam Brind profile imageAUTHOR

        Sam Brind 

        16 months ago

        Thanks for commenting, you are right that there is currently a lot of leeway. We definitely know something extra is there but not what it is, beyond ruling out the normal matter we are accustomed to.

      • profile image

        Setank Setunk 

        16 months ago

        These theories are looser than those regarding Black Holes; but at least we can actually see what can reasonably be called "Black Holes" in space. But there is something to the dynamic an energetic movement in our Universe. I am inclined to give them considerable leeway on these ideas. What about you?

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