Galaxies and Cosmology
4. Dark Matter and Dark Energy
Learning outcomes
- I can explain why dark matter was proposed.
- I can describe evidence for dark matter.
- I can explain the concept of dark energy.
- I can compare dark matter and dark energy.
- I can describe why these remain active areas of research.
The Invisible Universe
When astronomers observe the Universe, they can see stars, planets, galaxies, glowing gas, and other forms of ordinary matter.
However, observations show that the matter we can see does not appear to provide enough gravity to explain how galaxies and galaxy clusters behave.
Astronomers therefore proposed the existence of an additional form of matter called dark matter.
Later, observations revealed another mystery: the expansion of the Universe is accelerating. Scientists use the term dark energy for the unknown component associated with this accelerated expansion.
Together, dark matter and dark energy appear to make up most of the Universe, although their underlying nature remains uncertain.
What Is Dark Matter?
Dark matter is the name given to matter that does not appear to emit, absorb, or reflect enough electromagnetic radiation for us to detect it directly with ordinary telescopes.
We infer its presence mainly from its gravitational effects.
Dark matter was proposed because the gravity produced by visible matter alone cannot adequately explain several astronomical observations.
Importantly, scientists do not simply call something dark matter because they cannot see it. Dark matter is proposed because there are measurable gravitational effects that require an explanation.
Evidence for Dark Matter
Several independent observations provide evidence for the existence of dark matter.
1. Galaxy Rotation Curves
Stars orbit the centres of galaxies because of gravity.
If most of a galaxy's mass came from the visible stars and gas concentrated toward its centre, we would expect stars far from the centre to orbit considerably more slowly.
However, observations show that stars in the outer regions of many galaxies move much faster than expected.
A typical galaxy rotation curve looks approximately like this:
- Expected from visible matter: orbital speed decreases significantly with distance.
- Observed: orbital speeds remain unexpectedly high far from the centre.
One explanation is that galaxies are surrounded by large amounts of invisible mass forming a dark matter halo.
The gravity from this additional matter helps explain the observed orbital speeds.
2. Gravitational Lensing
According to general relativity, mass bends spacetime. As a result, light travelling near a massive object can have its path bent.
This effect is called gravitational lensing.
Astronomers can examine how much light from distant galaxies is distorted and use this information to estimate the mass of objects between the distant galaxy and Earth.
In many cases, the amount of mass inferred from gravitational lensing is much greater than the amount of visible matter.
This provides another way of detecting the gravitational influence of matter that cannot be seen directly.
3. Galaxy Clusters
Galaxies are often found together in enormous groups called galaxy clusters.
When astronomers study the motions of galaxies within these clusters, they find that the visible matter does not appear to provide enough gravitational attraction to explain the observed motions and keep the systems bound.
This suggests that galaxy clusters contain large quantities of additional unseen mass.
4. The Bullet Cluster
One particularly important observation comes from the Bullet Cluster, produced by the collision of two galaxy clusters.
During the collision:
- Much of the hot ordinary gas interacted and slowed down.
- The galaxies largely passed through one another.
- Gravitational lensing revealed that much of the system's inferred mass was located away from the hot gas.
This separation between ordinary matter and the inferred distribution of mass is an important piece of evidence supporting the existence of dark matter.
What Could Dark Matter Be?
Scientists still do not know exactly what dark matter is.
It does not appear to consist mainly of ordinary atoms such as those making up:
- Stars
- Planets
- Gas clouds
- Humans
Several possible dark matter particles have been proposed.
Scientists search for dark matter using:
- Underground particle detectors
- Particle accelerators
- Space telescopes
- Astronomical observations
So far, no proposed dark matter particle has been definitively identified.
What Is Dark Energy?
Dark matter is not the only major mystery in cosmology.
During the twentieth century, astronomers discovered that the Universe is expanding.
Galaxies are generally becoming farther apart as space itself expands.
For many years, scientists expected gravity to gradually slow this expansion.
Then, observations of distant exploding stars called Type Ia supernovae produced a surprising result.
The expansion of the Universe appears to be accelerating.
Scientists use the term dark energy to describe the unknown component associated with this accelerated expansion.
Evidence for Dark Energy
One major line of evidence came from observations of distant Type Ia supernovae.
Because these supernovae have predictable properties, astronomers can use them to estimate cosmic distances.
Measurements showed that distant supernovae appeared farther away than expected in a Universe whose expansion was simply slowing down.
The observations indicated that cosmic expansion has instead been speeding up during the more recent history of the Universe.
Other evidence, including measurements of the cosmic microwave background and the large-scale distribution of galaxies, also supports cosmological models containing dark energy.
What Causes Dark Energy?
Scientists do not yet know.
One leading explanation involves the cosmological constant, often represented by the Greek letter:
Λ
In this model, empty space itself has an energy density that remains approximately constant as the Universe expands.
Other possibilities have also been proposed, including changing forms of energy or modifications to our understanding of gravity.
At present, determining the physical nature of dark energy remains one of the major problems in modern physics.
What Is the Universe Made Of?
Current cosmological measurements indicate that the Universe is approximately:
5% ordinary matter 27% dark matter 68% dark energy
These values are approximate and depend somewhat on the cosmological model and observations used.
This produces a remarkable conclusion:
Everything made from ordinary atoms represents only about 5% of the total cosmic energy budget.
Stars, planets, gas, dust, and living organisms all belong to this small fraction.
Dark Matter vs Dark Energy
Despite their similar names, dark matter and dark energy describe very different phenomena.
| Feature | Dark Matter | Dark Energy |
|---|---|---|
| Directly observed? | No | No |
| Main evidence | Gravitational effects | Accelerating cosmic expansion |
| Important scale | Galaxies and galaxy clusters | Universe as a whole |
| Effect | Adds gravitational attraction and helps structure form. | Associated with accelerated expansion |
| Approximate cosmic fraction. | 27% | 68% |
| Nature understood? | No | No |
A useful distinction is:
Dark matter helps explain why galaxies and galaxy clusters behave as though they contain more mass than we can see.
Dark energy helps explain why the expansion of the Universe is accelerating.
Dark Matter and Galaxy Formation
Dark matter may have played an important role in producing the structure we see throughout the Universe.
In the early Universe, slightly denser regions containing dark matter attracted additional matter through gravity.
Over billions of years, these regions grew.
Ordinary matter collected within these gravitational structures, eventually contributing to the formation of:
- Galaxies
- Galaxy clusters
- Large-scale cosmic structures
Computer simulations containing dark matter reproduce many important features of the observed large-scale structure of the Universe.
Why Are They Called "Dark"?
The word dark does not simply mean black.
Instead, it reflects the fact that the underlying phenomena are not directly explained by ordinary luminous matter.
Dark matter is difficult to detect electromagnetically.
Dark energy is even more mysterious because scientists do not yet know what physical mechanism produces the observed acceleration.
In both cases, scientists have strong observational evidence for effects that require explanation, while the fundamental nature of the underlying cause remains uncertain.
Why Are These Active Areas of Research?
Dark matter and dark energy are among the largest unanswered questions in modern science.
Scientists are trying to determine:
What is dark matter?
Researchers are searching for possible new particles and testing alternative explanations for the gravitational observations.
What is dark energy?
Scientists are making increasingly precise measurements of cosmic expansion to determine whether dark energy behaves like a cosmological constant or changes over time.
Is our theory of gravity complete?
General relativity describes gravity extremely successfully, but scientists continue testing whether modifications might be necessary on enormous cosmic scales.
How has the Universe evolved?
Understanding dark matter and dark energy would help scientists explain how the Universe developed from its early state into the galaxies and large-scale structures observed today.
How Science Deals with the Unknown
Dark matter and dark energy provide excellent examples of how science operates.
Scientists do not need to know exactly what something is before they can investigate its effects.
The process often begins with observations that do not fit existing explanations.
Scientists then:
- Develop hypotheses and models.
- Determine what those models predict.
- Collect additional evidence.
- Compare observations with predictions.
- Modify or reject explanations when necessary.
Dark matter and dark energy remain accepted parts of the standard cosmological model because models containing them currently explain a wide range of observations very successfully.
However, their underlying physical nature remains an open scientific question.
Did You Know?
The term dark matter refers to something we cannot currently detect directly through ordinary electromagnetic observations, but scientists can map its inferred distribution using gravitational lensing.
In effect, astronomers can use the bending of light to investigate matter they cannot directly see.
Key Terms
- Dark matter – unseen matter inferred mainly from its gravitational effects.
- Dark energy – the name given to the unknown component associated with the accelerating expansion of the Universe.
- Galaxy rotation curve – a graph showing orbital speed at different distances from a galaxy's centre.
- Dark matter halo – the proposed large distribution of dark matter surrounding a galaxy.
- Gravitational lensing – bending of light caused by gravity.
- Galaxy cluster – a large gravitationally bound collection of galaxies.
- Type Ia supernova – a type of stellar explosion useful for measuring cosmic distances.
- Cosmological constant – a possible explanation for dark energy involving an approximately constant energy density of space.
- Cosmic expansion – the increase in distances between widely separated regions of the Universe as space expands.
Key Takeaways
- Dark matter was proposed because visible matter alone cannot explain several observed gravitational effects.
- Evidence for dark matter includes galaxy rotation curves, galaxy clusters, gravitational lensing, and observations such as the Bullet Cluster.
- Dark matter appears to contribute substantially to the formation and structure of galaxies.
- Observations indicate that the expansion of the Universe is accelerating.
- Dark energy is the name given to the unknown component associated with this acceleration.
- Dark matter and dark energy are not the same thing.
- Ordinary matter represents only about 5% of the cosmic energy budget in the standard cosmological model.
- Scientists have strong evidence for the phenomena attributed to dark matter and dark energy, but their fundamental nature remains unknown.
- Understanding them is one of the most important areas of research in modern astronomy and physics.