The Invisible Universe
Everything we can see (stars, planets, gas, people) makes up only about 5 % of the universe. The rest is made of two things we cannot see directly and do not yet understand.
| Component | Share of the universe | What it does |
|---|---|---|
| Ordinary matter (atoms) | about 5 % | Everything we know |
| Dark matter | about 27 % | Adds extra gravity and holds galaxies together |
| Dark energy | about 68 % | Makes the expansion of the universe speed up |
(Values from the Planck satellite, 2018)
Dark Matter
What It Is
Dark matter has mass and gravity, but it gives off, absorbs and reflects no light. We only notice it through its gravitational pull.
The Evidence
| Observation | What it shows |
|---|---|
| Galaxy clusters (Fritz Zwicky, 1933) | Galaxies in the Coma Cluster move far too fast to be held together by the visible mass alone |
| Rotation curves (Vera Rubin, 1970s) | Stars at the edges of galaxies orbit just as fast as stars closer in. By Orbital Mechanics: Key Equations they should be much slower, unless there is a lot of invisible mass |
| Gravitational lensing | Galaxy clusters bend light more strongly than their visible mass can explain (see Einstein's Theory of Relativity) |
| Bullet Cluster | In this collision of two galaxy clusters, the hot gas (most of the ordinary mass) and the centre of the gravity are in different places. This is strong evidence for dark matter rather than for modified laws of gravity |
| Cosmic microwave background | The pattern of hot and cold spots in the leftover radiation from the Big Bang only fits when dark matter is included |
What Could It Be?
| Candidate | Idea | Status |
|---|---|---|
| WIMPs (weakly interacting massive particles) | Heavy particles that barely interact with normal matter | Searched for over decades in underground detectors such as XENONnT and LZ, not found yet |
| Axions | Extremely light particles | Searched for in experiments such as ADMX |
| Primordial black holes | Black holes formed shortly after the Big Bang | Largely ruled out for most mass ranges (see Black Holes) |
| MOND (modified gravity) | No dark matter; gravity itself works differently at very low accelerations | Explains rotation curves well, but struggles with galaxy clusters and the Bullet Cluster |
Dark Energy
The Discovery
In 1998, two teams studied distant exploding stars (type Ia supernovae). They expected the expansion of the universe to be slowing down because of gravity. Instead, they found that it is speeding up. This earned Perlmutter, Schmidt and Riess the 2011 Nobel Prize in Physics.
What Is It?
The simplest explanation is Einstein's cosmological constant (Λ): an energy that belongs to empty space itself, with the same density everywhere and at all times. As the universe grows, there is more space, so there is more dark energy, while matter becomes more and more spread out.
The standard model of cosmology is therefore called ΛCDM ("Lambda Cold Dark Matter").
The Big Problem
When quantum physics is used to estimate the energy of empty space, the answer is about 10¹²⁰ times larger than what is measured. This is often called the worst prediction in the history of physics.
Open Questions Today
- Hubble tension: different methods give different values for how fast the universe is expanding today. Measurements of the early universe give about 67 km/s per megaparsec; measurements of the nearby universe give about 73. The cause is still unknown.
- Does dark energy change over time? Results from the DESI survey published in 2024 and 2025 hint that dark energy might be getting weaker. This would mean it is not a simple constant. The evidence is not yet conclusive.
- New observatories: the Euclid space telescope (launched 2023) and the Vera C. Rubin Observatory map billions of galaxies to measure dark matter and dark energy more precisely.
What It Means for the Future
| If dark energy… | Then the universe… |
|---|---|
| stays constant | keeps expanding faster and faster. Distant galaxies vanish from view, and it ends cold and empty (see The Laws of Thermodynamics, heat death) |
| gets stronger | could end in a "Big Rip" that tears apart even atoms |
| gets weaker or reverses | could slow down, or even collapse again in a "Big Crunch" |