Dark Matter and Dark Energy Explained: The Hidden Structure of the Universe
Understanding the Universe: Dark Matter, Dark Energy, and Cosmic Structure
What Is the Universe Made Of?
Understanding the structure of the universe is one of the most complex challenges in science. Its immense scale and three-dimensional structure can be difficult to visualise through written descriptions, images, or videos alone. However, a few simple examples can help us understand its fundamental concepts.
To explore the universe, we first need to understand ordinary matter, dark matter, and dark energy—and how these three components differ.
Ordinary Matter: The Part of the Universe We Know
The things we see and touch around us are made of ordinary matter. Earth, plants, humans, rocks, water, air, the Sun, and other stars are all examples. Most ordinary matter is made up of atoms.
Imagine standing beside a friend. The space between you may appear empty, but the air around you contains molecules, including nitrogen, oxygen, and carbon dioxide. It may also contain water vapour, microscopic dust particles, and other substances.
In other words, space that appears empty to our eyes is not necessarily completely empty.
Earth’s atmosphere is essential for life. It supplies the air we breathe, influences temperatures, and helps protect us from certain harmful forms of solar radiation. However, the atmosphere gradually becomes thinner as altitude increases, eventually giving way to the extremely sparse environment of space.
Matter also exists in space, including gas, dust, radiation, stars, planets, and other celestial objects. Yet most of the universe is extraordinarily diffuse compared with Earth’s atmosphere.
This raises an important question: Is everything in the universe made of the same kind of matter that we can see and study directly?
The answer is no.
Why Is Approximately 95% of the Universe Still a Mystery?
According to the standard cosmological model, ordinary matter accounts for approximately 5% of the universe’s total matter-energy content. Dark matter accounts for roughly 27%, while dark energy contributes approximately 68%.
These figures are estimates derived from our current cosmological model. They do not mean that the remaining 95% exists in one enormous empty region, nor do they mean that scientists have directly observed all of it.
Instead, these estimates are based on astronomical observations and models involving the universe’s expansion, gravitational effects, galaxy movements, and other measurable phenomena.
The greatest challenge is that we still do not fully understand the fundamental nature of dark matter and dark energy. There is substantial observational evidence for the effects associated with them, but their underlying physical nature remains an active area of scientific research.
What Is Dark Matter?
To understand dark matter, we need to examine gravity and the movement of galaxies.
A galaxy can contain billions of stars, along with gas, dust, and other celestial objects. All these components are affected by gravity. When scientists measure the movement of stars and gas within galaxies, they often find that the observed motions are difficult to explain using only the gravity of visible matter.
If galaxies contained only the matter we can see, their gravitational effects and the movements of their stars would be expected to follow particular relationships. However, observations of many galaxies reveal evidence of additional gravitational influence.
Where does this additional gravitational effect come from?
One leading explanation is that galaxies and their surroundings contain substantial amounts of matter that do not emit, reflect, or absorb light in ways that allow us to observe them directly. This unseen matter is called dark matter.
Evidence for dark matter does not come from galaxy rotation alone. Another important source is gravitational lensing: the bending of light from distant objects by the gravity of massive structures, such as galaxy clusters. These observations indicate that additional, unseen mass is present.
Dark matter is therefore not simply an imaginary concept introduced for convenience. Multiple independent observations support the existence of its gravitational effects. Nevertheless, scientists have not yet established exactly what dark matter is made of or identified its fundamental particles.
Why Don't Galaxies Fly Apart?
Consider a simple example.
Imagine a spinning disc with several small objects resting on it. If those objects are not held securely, they may slide or fly off as the disc rotates rapidly.
Now compare this example with a galaxy. Stars orbit the galaxy’s centre, influenced by gravity. In many galaxies, the observed speeds of stars—particularly those farther from the centre—are difficult to explain using only the gravity of visible matter.
However, a galaxy is not a solid spinning disc, and its stars are not connected by physical strings. Gravity is the key factor governing their motion. The presence of dark matter helps explain the galaxy’s total gravitational influence.
Put simply, if an object contains more mass than we can account for through visible material alone, its gravitational effects may be stronger than our initial calculations suggest. Dark matter is a leading explanation for this additional mass.
This does not mean every galaxy maintains its structure solely because of dark matter. Ordinary matter also contributes to gravity, and scientists study the distribution of mass, stellar motion, and other factors to understand how galaxies behave.
The Cosmic Web: Clusters, Filaments, and Voids
When we examine the universe on the largest scales, galaxies are not distributed uniformly. Instead, they form clusters, long interconnected structures, and enormous regions where relatively few galaxies are found.
Groups of galaxies are known as galaxy clusters when they form sufficiently large gravitationally bound systems. Galaxies and galaxy clusters are arranged within an even larger network of structures called the cosmic web.
One way to visualise this arrangement is to imagine the branches of a tree. Just as branches and smaller twigs spread out and support clusters of leaves, galaxies and galaxy clusters are concentrated in certain regions of the universe. Between these regions lie vast areas containing relatively few galaxies. These are called cosmic voids.
This is only an analogy. The universe is not literally a tree, and the cosmic web is not made of physical branches. It is a large-scale structure that developed through gravity and the distribution of matter.
Dark matter plays an important role in explaining how this structure formed. According to current cosmological models, its gravitational influence helped ordinary matter gather into increasingly dense regions, contributing to the formation of galaxies and larger cosmic structures over time.
What Is Dark Energy?
If ordinary matter and dark matter together account for approximately 32% of the universe’s total matter-energy content, what accounts for the remaining 68%?
In the standard cosmological model, this component is associated with dark energy.
Despite their similar names, dark matter and dark energy represent different concepts. Dark matter’s gravitational influence helps explain how matter gathers into structures such as galaxies. Dark energy, by contrast, is the name given to the component or effect associated with the accelerating expansion of the universe.
In the late twentieth century, observations of distant supernovae revealed that the universe’s expansion is accelerating. The universe was not merely continuing to expand; the rate of expansion was increasing. This discovery raised a fundamental question: What is causing this acceleration?
The concept of dark energy became important in attempts to explain the observations.
However, scientists have not directly observed dark energy as a separate object or isolated it in a laboratory. Its effects are inferred from cosmological observations and theoretical models.
What is dark energy actually made of? Could it be related to the energy of empty space, a changing field, or an aspect of gravity that we do not yet fully understand?
Scientists are still investigating these possibilities. Its true physical nature remains unknown.
Understanding Cosmic Expansion Through a Balloon Analogy
After distinguishing dark matter from dark energy, we can use a simple model to visualise the expansion of the universe.
Imagine creating a small model tree with flowers attached to its branches. Place this model inside a transparent balloon and begin inflating it.
As the balloon expands, the distance between markings on its surface increases. If the flowers are represented by markings on that surface, they appear to move farther apart even though they are not travelling across the surface independently.
This analogy helps illustrate cosmic expansion. The distances between faraway galaxies can increase without those galaxies simply flying through pre-existing space like fragments of an ordinary explosion. On large scales, space itself expands.
In this example, the flowers represent galaxies or galaxy clusters. However, it would not be accurate to identify the tree branches directly with dark matter or the air inside the balloon directly with dark energy. Dark matter is associated with gravity and the formation of cosmic structure, whereas dark energy is associated with the accelerating expansion of the universe.
The balloon analogy also does not represent the universe’s actual geometry or explain every aspect of its expansion. Its purpose is simply to show why increasing cosmic distances do not require every galaxy to fly away from a single central point.
Is Everything in the Universe Constantly Moving?
Stars, planets, galaxies, and other celestial objects are in motion. Planets orbit stars, stars move within galaxies, and galaxies move relative to one another within larger cosmic structures.
In addition, the expansion of the universe increases the distances between faraway galaxies on large scales.
However, this does not mean that every object is always moving away from every other object. Structures held together by gravity—such as solar systems and many systems within galaxies—do not expand in the same way as the universe on the largest scales.
To understand cosmic motion, we must distinguish between two processes: the movement of celestial objects through space and the expansion of space itself over large distances.
Conclusion: Most of the Universe Remains Unexplained
Ordinary matter, dark matter, and dark energy are central to our current understanding of the universe.
Ordinary matter forms stars, planets, and the familiar world around us. Dark matter’s gravitational effects help explain the behaviour of galaxies and the formation of large-scale cosmic structures. Dark energy is a leading component of the standard cosmological model used to explain the accelerating expansion of the universe.
Yet the two components associated with approximately 95% of the universe’s total matter-energy content remain poorly understood at a fundamental level.
This is one of the most important aspects of science: it does not claim to possess every answer. Scientists make observations, examine evidence, develop models, and refine their understanding when new information becomes available.
The universe is immense, and our knowledge of it remains incomplete. Future telescopes, more precise measurements, and new experiments may bring us closer to understanding the mysteries we currently call dark matter and dark energy.
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