Dark Matter and Dark Energy

This lesson covers dark matter and dark energy — two of the most mysterious and important concepts in modern physics — as required by the AQA GCSE Physics specification (4.8.2). This is a Physics-only topic. You need to understand why scientists believe dark matter and dark energy exist, what evidence supports their existence, and how they affect the universe.

---

The Missing Mass Problem

In the 1930s, the Swiss astronomer Fritz Zwicky studied clusters of galaxies and noticed something puzzling. When he calculated the total mass of a galaxy cluster by adding up the masses of all the visible stars, gas, and dust, the total was far too small to account for the gravitational forces holding the cluster together. The galaxies were moving so fast that they should have flown apart — yet the cluster remained bound together.

Zwicky proposed that there must be a large amount of invisible matter providing the extra gravitational pull needed to hold the cluster together. He called this unseen material "dark matter" (dunkle Materie in German).

Exam Tip: The key reason scientists proposed dark matter is that the observed mass of galaxies and galaxy clusters is not enough to account for the gravitational effects observed. The visible matter alone cannot explain the orbital speeds of stars in galaxies or the motion of galaxies within clusters.

---

Evidence for Dark Matter

Galaxy Rotation Curves

The strongest evidence for dark matter comes from the rotation curves of galaxies. When astronomers measure how fast stars orbit the centre of a spiral galaxy at different distances, they find:

• Stars far from the centre of the galaxy orbit much faster than expected based on the visible mass of the galaxy.
• If only the visible matter existed, stars at the outer edges of galaxies should orbit slowly (like the outer planets in our solar system orbit the Sun more slowly than the inner planets).
• Instead, the orbital speeds remain roughly constant even at great distances from the centre.

This can only be explained if there is a large amount of unseen mass extending well beyond the visible edges of the galaxy — a dark matter halo.

Distance from Galaxy Centre	Expected Speed (visible matter only)	Observed Speed
Close to centre	High	High
Middle	Moderate	Roughly constant
Far from centre	Low (decreasing)	Roughly constant (not decreasing)

Gravitational Lensing

Gravitational lensing occurs when the gravitational field of a massive object (such as a galaxy cluster) bends the path of light from more distant objects behind it. The amount of bending depends on the total mass of the lensing object.

When astronomers measure gravitational lensing by galaxy clusters, the bending is much greater than can be accounted for by the visible mass alone. This provides further evidence for the existence of dark matter.

Other Evidence

• The CMBR has tiny fluctuations that are best explained by models that include dark matter.
• The way large-scale structures (galaxy clusters, filaments, and voids) formed in the universe is consistent with dark matter providing the gravitational scaffolding.

Exam Tip: For the exam, you should be able to state that galaxy rotation curves provide evidence for dark matter. The key point is: stars at the edges of galaxies orbit faster than expected, which can only be explained by additional unseen mass (dark matter). You do not need to explain gravitational lensing in detail, but it is worth knowing as a second piece of evidence.

---

What Is Dark Matter?

Despite decades of research, scientists still do not know what dark matter is made of. However, we know several things about it:

Property	Description
Does not emit light	Dark matter does not produce, absorb, or reflect electromagnetic radiation (light, radio waves, X-rays, etc.) — it is invisible
Has mass	Dark matter exerts gravitational force on visible matter, bending light and affecting the motion of stars and galaxies
Does not interact with normal matter (except through gravity)	Dark matter particles pass through ordinary matter without interacting electromagnetically
Makes up about 27% of the universe	Visible (ordinary) matter makes up only about 5% of the total mass-energy of the universe

Scientists have proposed several candidates for dark matter particles, including:

• WIMPs (Weakly Interacting Massive Particles) — hypothetical particles that interact through gravity and the weak nuclear force but not electromagnetism
• Axions — very light hypothetical particles
• MACHOs (Massive Astrophysical Compact Halo Objects) — large objects like black holes or brown dwarfs, though these alone cannot account for all dark matter

Experiments around the world are searching for dark matter particles, but none have been conclusively detected yet.

---

The Accelerating Expansion of the Universe

In the 1990s, two independent teams of astronomers studying distant Type Ia supernovae made a startling discovery: the expansion of the universe is not slowing down, as everyone expected — it is accelerating.

This was surprising because:

• Gravity should be pulling galaxies together, slowing the expansion.
• For the expansion to be accelerating, there must be a mysterious repulsive force pushing galaxies apart — overcoming gravity on the largest scales.

This mysterious force was named dark energy.

graph TD
    A["Universe Composition"] --> B["Ordinary (Visible) Matter: ~5%"]
    A --> C["Dark Matter: ~27%"]
    A --> D["Dark Energy: ~68%"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#8e44ad,color:#fff
    style D fill:#e74c3c,color:#fff

Exam Tip: The composition of the universe is a common exam question. Remember: about 5% ordinary matter, about 27% dark matter, and about 68% dark energy. This means that the matter we can see (stars, planets, gas, dust) makes up only about 5% of the total mass-energy content of the universe.

---

What Is Dark Energy?

Dark energy is the name given to the unknown form of energy that is causing the expansion of the universe to accelerate. Like dark matter, we do not fully understand what dark energy is, but we know its effects.

Property	Description
Repulsive effect	Dark energy acts as a repulsive force, pushing the universe apart at an accelerating rate
Uniform distribution	Dark energy appears to be spread evenly throughout space
Makes up about 68% of the universe	It is the dominant component of the universe's total energy
Not fully understood	Dark energy is one of the greatest unsolved problems in physics

Possible Explanations

Several ideas have been proposed to explain dark energy:

1. Cosmological constant — Einstein originally included a "cosmological constant" in his equations of general relativity to keep the universe static. He later called it his "biggest blunder," but the discovery of dark energy has revived this idea. The cosmological constant represents a constant energy density filling all of space.

2. Quintessence — a hypothetical dynamic energy field that changes over time and space.

3. Modified gravity — perhaps our understanding of gravity needs to be modified on the largest scales.

None of these explanations has been conclusively confirmed.

---

The Importance of Dark Matter and Dark Energy

Together, dark matter and dark energy account for about 95% of the total mass-energy of the universe. This means that all the ordinary matter we can see — stars, planets, gas, dust, galaxies — makes up only about 5% of the universe.

Understanding dark matter and dark energy is one of the greatest challenges in modern physics. Their existence tells us that our current understanding of the universe is incomplete and that there is much more to discover.