What is Dark Matter?

A simulation of the dark matter distribution in the universe 13.6 billion years ago

Dark matter is a type of matter which makes up roughly 80% of the entire matter content in the universe. Unlike ordinary matter, dark matter does not emit, absorb, or reflect light, making it impossible to see and extremely hard to detect.

Without the inclusion of dark matter, the expansion of the universe would cause the abundance of ordinary matter to expand beyond the influence of gravity. This means that no stars or planets would have formed resulting in the universe remaining a cold, dark and lifeless place.

As well as allowing us to live, dark matter allows scientists to make accurate predictions of how the universe evolves over time. This is done by creating dark matter simulations of the universe and comparing them with the observational universe. However, simulating an isolated universe dominated by dark matter does not yield results exactly like observations. This is due to other factors in the universe such as energy from evolving stars and massive black holes known as “feedback”. It is key to incorporate feedback into the simulation in order to produce data that resembles our observed universe.

However some scientists do not believe in the existence of dark matter, and instead suggest that our understanding of gravity is incomplete. This would result in modifying Einstein’s theory of general relativity to re-describe how gravity behaves.

In order to determine which proposal is correct, experimental evidence is needed!

Contents of the Universe

Pi Chart of the content of the universe. Dark Matter makes up 26.6% of the entire content of the universe, which is roughly 80% of the matter content of the universe.

	Percentage	Label
	4.9%	Ordinary Matter
	68.5%	Dark Energy
	26.6%	Dark Matter

Evidence for Dark Matter

Rotation curve of a typical galaxy. The dashed blue curve (A) is the predicted motion of the galaxy meanwhile the solid red curve (B) is the observed motion. This discrepancy between the curves is due to dark matter.

Galaxy Rotation Curves

A fundamental difference between star clusters and galaxies lies in the influence of dark matter. In star clusters, stars orbiting further from the centre move slower than those in inner orbits, similar to the behaviour observed in our solar system. For galaxies, this pattern was also expected. However, observations showed that the rotational speed of stars remained constant at greater distances from the galactic centre instead of decreasing, as illustrated in Figure 2. To account for this constant velocity, scientists inferred that there must be more mass present in the outer regions of galaxies than what was visible. This discrepancy led to the proposal of an invisible substance called “dark matter”.

Galaxy clusters: The Coma Cluster

One of the first pieces of evidence for the potential existence of dark matter was found when observing galaxy clusters. These are structures made up of hundreds of galaxies bound together by gravity, permeated with a mixture of hot, interstellar gas known as the Intra-Cluster Medium (ICM). The Coma galaxy cluster, which is only 300 million light years away, is one of the largest known clusters. Coma and many other similar galaxies were observed in 1933 by the Swiss astronomer F. Zwicky who aimed to catalogue their average velocities. From this, it was found that its density was much larger than what was expected from calculations using luminosity data. This discrepancy suggests the presence of an additional unseen mass, acting as a strong indicator for the presence of dark matter.

Gravitational Lensing

Gravitational lensing is a phenomenon predicted by Einstein’s general theory of relativity, where the gravitational field of a massive object, such as a galaxy, bends the path of light travelling from a more distant object. This bending of light can magnify, distort, and multiply images of the distant light source.

Using gravitational lensing, scientists can calculate the mass of a massive object by measuring the degree of light distortion it causes. However, calculations revealed that more mass was present than previously estimated, indicating the presence of dark matter.

Galactic clusters: The Bullet Cluster

The search for dark matter through gravitational lensing led to the study of another galaxy cluster. The Bullet Cluster consists of two smaller sub-clusters approximately 3.8 billion light-years away that collided in one of the highest-energy events in the universe since its creation. During the collision, the ICM within each cluster, which contained the vast majority of baryonic (normal) matter, slowed down due to friction. In contrast, the galaxies passed through each other relatively unimpeded due to their large separation. This created a discrepancy between the visible mass of the cluster and the mass inferred from gravitational lensing studies.

Gravitational lensing revealed that the highest concentration of mass in the cluster is not centred around the hot gas but rather around the galaxies, which had moved ahead of the gas during the collision. This observation implies that dark matter, unlike the gas, was not subject to friction during the collision. Instead, it remained aligned with the galaxies, reinforcing the idea that dark matter interacts weakly with itself and other particles. This provides some of the strongest evidence to date for the existence of dark matter.

Cosmic Microwave background radiation:

The Cosmic Microwave Background (CMB) radiation is the thermal radiation left over from the Big Bang, permeating the entire universe. Data collected by satellites such as the Planck satellite revealed that the CMB exhibits small temperature fluctuations. These fluctuations are attributed to slight density variations in the early universe, which are best explained by the presence of dark matter.