For centuries, humans have looked up at the night sky and wondered what the Universe is made of. Ancient astronomers could only see a few thousand stars with the naked eye, but now we know there are hundreds of billions of galaxies each containing hundreds of billions of stars. Yet when scientists measure all the “normal” matter we can see, it only accounts for 4% of the total matter and energy in the Universe. So what is the other 96% of the Universe made of?
History of Understanding the Composition of the Universe
Our understanding of what makes up the Universe has evolved dramatically over the centuries as our knowledge and technology have improved.
Pre-20th Century Ideas
For most of human history, people thought the Universe consisted entirely of the Earth, Sun, Moon, planets, and stars visible to the naked eye. This was the prevailing Aristotelian cosmology for nearly 2000 years. Copernicus and Galileo’s heliocentric model expanded ideas about the contents of the Universe in the 16th and 17th centuries. Still, the Universe was assumed to be entirely made up of the matter we observe around us.
Discovery of Atoms and Subatomic Particles
In the 19th and early 20th centuries, scientists discovered that ordinary matter is made up of chemical elements like hydrogen, oxygen, carbon, and iron. Further work revealed these elements are comprised of atoms with nuclei surrounded by electrons. The nucleus contains protons and neutrons, which can be further divided into quarks and gluons. This demonstrated the Universe contains previously unknown types of subatomic particles.
Detection of Dark Matter
In the 1930s, astronomers analyzing the motion of galaxies and galaxy clusters realized there must be much more matter in the Universe than what we can see. They termed this invisible matter “dark matter.” Dark matter does not emit, reflect, or absorb light, but it does exert gravitational influence. By the 1970s, scientists determined dark matter makes up about 25% of the Universe.
Contents of the Universe
Our current best understanding based on multiple independent lines of evidence is that the Universe consists of:
- 4% Ordinary matter – Atoms, subatomic particles, gas, stars, planets, etc.
- 25% Dark matter – Unknown non-luminous matter detectable only by its gravitational effects.
- 70% Dark energy – Mysterious energy inherent to space which causes the Universe’s expansion to accelerate.
Let’s examine what each of these components consists of in more detail:
Ordinary Matter – 4%
Ordinary matter, also known as baryonic matter, refers to any matter made up of protons, neutrons, and electrons. This includes:
- Atoms and molecules
- Subatomic particles like protons, neutrons, and electrons
- Gases, dust, and plasma between stars and galaxies
- Stars, planets, asteroids, comets, and other celestial bodies
- Any matter that interacts with electromagnetic radiation
All the matter we can physically interact with and observe in telescopes or particle detectors falls into this category. But it is only a small fraction of the overall Universe’s contents.
Dark Matter – 25%
The existence of dark matter is inferred from observations of gravitational effects that cannot be accounted for by visible matter. We know dark matter exists, but it does not interact via electromagnetic forces. Key evidence and properties of dark matter include:
- Galaxy rotation curves – Stars at the edges of spiral galaxies orbit faster than expected unless extra invisible matter is present.
- Gravitational lensing – Light from distant objects is bent more than anticipated, indicating extra mass between the object and Earth.
- Cosmic microwave background – Variations in the CMB indicate the presence of dark matter starting in the early Universe.
- Galaxy cluster masses – The total mass of galaxy clusters deduced from gravitational interactions exceeds the mass of visible galaxies.
- Does not emit, reflect, or absorb light (otherwise we would be able to see it).
- Interacts via gravity, but no other known forces.
- May consist of unknown subatomic particles like WIMPs (Weakly Interacting Massive Particles).
Dark Energy – 70%
Dark energy is the most mysterious component of the Universe. While dark matter exerts gravitational attraction, dark energy causes a repulsive effect driving the Universe’s accelerating expansion. Discovered in 1998, key properties of dark energy include:
- Causes the Universe to expand at an accelerating rate, rather than slowing down due to gravity.
- Inferred from observations of distant supernovae and the cosmic microwave background.
- Believed to be a fundamental energy inherent to space itself, represented by the cosmological constant term in Einstein’s equations.
- Has hugely negative pressure, counteracting the attractive force of gravity.
- Remains constant across space and time (does not dilute as space expands).
The physical nature and origin of dark energy remains one of the biggest mysteries in physics. It may be related to quantum fluctuations in the fabric of spacetime, vacuum energy, or other exotic physics.
Summary of Universal Composition
This table summarizes our current model of what the Universe consists of:
| Component | Percentage | Properties |
|---|---|---|
| Ordinary Matter | 4% | Atoms, subatomic particles, gas, stars, planets, etc. Interacts with light and gravity. |
| Dark Matter | 25% | Invisible matter detectable only by gravitational effects. Does not interact with light. |
| Dark Energy | 70% | Mysterious energy inherent to space causing accelerating expansion. Negative pressure counteracting gravity. |
Remaining Questions and Mysteries
Our current Lambda-CDM model summarizing the contents of the Universe provides a remarkably good fit to observational data. However, there are still many open questions remaining about the true nature of dark matter and dark energy:
- What exactly is the dark matter particle? Are WIMPs or some other exotic particle responsible?
- Are there alternative gravitational theories that can explain dark matter effects without requiring new particles?
- Is dark energy truly constant, or does it evolve over cosmic timescales?
- Is dark energy directly related to the quantum vacuum or an emergent cosmological property?
- Was the predominance of matter over antimatter in the early Universe related to dark matter?
- Are there additional unseen components besides dark matter and dark energy?
Unraveling these mysteries may require revolutionary new physics and astronomy. Upcoming experiments like the Large Hadron Collider, James Webb Space Telescope, and Extremely Large Telescope will shed new light by hunting for dark matter particles, probing dark energy over cosmic time, and searching for primordial antimatter. Solving the enigma of the dark Universe remains one of the most exciting frontiers in modern cosmology and particle physics.
Conclusion
To summarize, the current best cosmological model indicates that:
- 4% of the Universe consists of ordinary matter – atoms, subatomic particles, gas, stars, planets, etc.
- 25% is dark matter – an unknown matter component detected only by its gravitational effects.
- 70% is dark energy – a mysterious negative pressure energy inherent to space driving the Universe’s accelerating expansion.
Explaining the observed effects ascribed to dark matter and dark energy in terms of fundamental physics remains an outstanding challenge for modern science. Unraveling these mysteries may uncover new particles, forces, quantum effects, and radical new laws of cosmology. Determining the ultimate nature of the invisible 96% of the Universe is key to developing a complete understanding of the cosmos.
