Maarg Manthan · Topic 9.3

Dark Matter and Dark Energy

Science & Technology › Physics – Modern Physics and Astrophysics · Topic 9.3

Dark matter and dark energy are two unseen components that make up most of the universe. Ordinary matter, which forms stars, planets and people, is only about 5 per cent of the total content. Dark matter is about 27 per cent, and dark energy is about 68 per cent, according to measurements of the cosmic microwave background by the Planck satellite. Dark matter does not emit, absorb or reflect light, but its gravity holds galaxies and clusters together. Dark energy is a mysterious effect that makes the expansion of the universe speed up, found in 1998 by two teams studying distant supernovae, for which Saul Perlmutter, Brian Schmidt and Adam Riess received the Nobel Prize in 2011. James Peebles received the Nobel Prize in 2019 for work on the evolution of the universe.

5 min read · UPSC Prelims · Science & Technology · Physics – Modern Physics and Astrophysics

At a Glance

Ordinary matter About 5 per cent
Dark matter About 27 per cent
Dark energy About 68 per cent
Acceleration found 1998; Nobel 2011
Space mission ESA Euclid (2023)
Exam link UPSC Prelims, Science & Technology: Physics – Modern Physics and Astrophysics

Where Does It Fit?

This is the third topic of the macro on modern physics and astrophysics. After the Big Bang, it addresses what the universe is made of and how it will evolve. Previous topic: Big Bang Theory and Cosmological Singularity. Next topic: Black Holes.

What Is the Evidence for Dark Matter?

Evidence	Explanation
Galaxy clusters	Fritz Zwicky found in 1933 that galaxies in the Coma cluster move too fast to be held by the visible matter alone
Rotation curves	Vera Rubin showed in the 1970s that stars at the edge of galaxies orbit as fast as those near the centre, which needs extra unseen mass
Gravitational lensing	Mass bends light, and the bending around clusters is much more than visible matter can explain
Bullet Cluster	In a colliding pair of clusters the mass found by lensing is separate from the hot gas, which suggests that dark matter is a different kind of matter
Cosmic microwave background	Patterns in the early radiation need a large amount of matter that does not interact with light
Structure formation	Galaxies could not form so quickly without the extra gravity of dark matter

What Is Dark Energy and What Might Dark Matter Be?

Accelerating expansion: in 1998, the Supernova Cosmology Project and the High-Z Supernova Search Team found that distant exploding stars were fainter than expected, showing that the expansion is speeding up.
Cosmological constant: Einstein introduced it in 1917 and later dropped it. It now serves as the simplest model of dark energy, a constant energy of empty space.
Other ideas: dark energy might change with time. Recent surveys, such as the Dark Energy Spectroscopic Instrument, have hinted at changes, but these results need confirmation.
Dark matter candidates: weakly interacting massive particles, axions and sterile neutrinos are proposed, while primordial black holes are another idea.
Not antimatter or black holes: dark matter is not made of ordinary matter, dust or antimatter, and it does not seem to be mostly black holes.
Peebles: James Peebles, who shared the 2019 Nobel Prize, helped to build the theory of cold dark matter and the cosmic microwave background.

How Are They Searched For and What Else Should You Remember?

Underground detectors: experiments such as XENONnT in Italy and LUX-ZEPLIN in the United States look for rare collisions of dark matter particles with atoms, deep underground to avoid cosmic rays.
Colliders: the Large Hadron Collider may produce dark matter particles, which would escape detection and show up as missing energy, as in Fundamental Particles and Basic Forces of Nature.
Space missions: the European Space Agency’s Euclid telescope, launched in 2023, maps billions of galaxies in 3D to study dark matter and dark energy, and NASA’s Nancy Grace Roman Space Telescope is planned for the same goal.
Telescopes and surveys: the Vera Rubin Observatory and the Dark Energy Survey map the sky, as in Space Observatories and Telescopes (Hubble, Chandra X-Ray Observatory, Thirty Metre Telescope, Spitzer Space Telescope).
Neutrinos: they are not enough to make up dark matter, but studies in underground labs such as the India-Based Neutrino Observatory plan to explore their properties.
Fate of the universe: if dark energy stays constant, the universe will expand forever and galaxies will drift apart.
Modified gravity: some scientists suggest changing the law of gravity instead of adding dark matter, but this has difficulty in explaining all the evidence.
Exam pattern: percentages of the universe, evidence and the 1998 discovery.

Why Does It Matter for the Exam?

Number questions: the shares of ordinary matter, dark matter and dark energy.
Concept questions: difference between dark matter and dark energy.
Current affairs link: Euclid images, new surveys and Nobel Prizes.
Link to the Big Bang: the standard model of cosmology needs both components, as in Big Bang Theory and Cosmological Singularity.
Common errors: saying that dark matter is the same as dark energy or that either has been directly detected.

As an analytical point, most of the universe is unknown. Dark matter pulls things together and dark energy pushes space apart, and identifying them is one of the great tasks of physics.

Why Is This Topic Important?

Facts Shares and discoveries asked
Concept Pull versus push
News Euclid and surveys

What Came Next?

The next topic covers black holes. Next topic: Black Holes.

Key People and Terms

Dark matter Unseen matter that has gravity
Dark energy Cause of accelerating expansion
Rotation curve Orbital speed against distance
Lensing Bending of light by mass
WIMP Candidate dark matter particle
Cosmological constant Constant energy of space

Exam Corner

Points to Remember

5% matter, 27% dark matter, 68% dark energy.
Acceleration found in 1998.
Zwicky: 1933; Rubin: 1970s.
Euclid: launched 2023.
Peebles: Nobel 2019.

Do Not Confuse With

Dark matter and dark energy Gravity that pulls versus effect that pushes space apart
Dark matter and antimatter Unseen versus opposite-charge matter
Lensing and redshift Bending of light versus stretching of wavelength

Memory Hook

Five, twenty-seven, sixty-eight: matter, dark matter, dark energy; Zwicky, Rubin, lensing prove the pull; nineteen ninety-eight the push.

Mains Angle

Questions ask about the unknown content of the universe. These points add depth.

Evidence Rotation, lensing
Dark energy Acceleration
Search Detectors and telescopes

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between dark matter and dark energy and discuss the evidence for their existence and the efforts to detect them.”

How to Answer

Share 5, 27, 68.
Matter Evidence.
Energy Acceleration.
Search Detectors.

GS Relevance

Prelims: shares of the universe, evidence, detectors and missions, Nobel Prizes. GS Paper 3: science and technology.

Frequently Asked Questions

What is dark matter?

Dark matter is a form of matter that does not emit, absorb or reflect light, so it cannot be seen, but its gravity affects the motion of stars and galaxies and bends light. It makes up about 27 per cent of the universe, and its nature is not yet known, with particles such as WIMPs and axions being proposed.

What is dark energy?

Dark energy is a mysterious form of energy, which makes up about 68 per cent of the universe, that causes the expansion of the universe to speed up. It was discovered in 1998 from the study of distant supernovae, and its nature is unknown, with the cosmological constant being the simplest explanation.

What is the evidence for dark matter?

The evidence includes the fast motion of galaxies in clusters, found by Fritz Zwicky in 1933, the flat rotation curves of spiral galaxies, studied by Vera Rubin, the bending of light by gravitational lensing, the Bullet Cluster and the patterns of the cosmic microwave background.

What is the Euclid mission?

Euclid is a space telescope of the European Space Agency, launched in 2023, that is mapping the shapes and positions of billions of galaxies out to a distance of about 10 billion light years. These maps will help scientists to study the distribution of dark matter and the effect of dark energy on the expansion of the universe.

What is the percentage of ordinary matter in the universe?

Ordinary matter, which forms stars, planets, gas and living things, is only about 5 per cent of the total mass and energy of the universe. The rest is dark matter at about 27 per cent and dark energy at about 68 per cent, according to measurements by the Planck satellite.

PYQ Practice — Statement Analysis

1 Dark energy makes up about 68 per cent of the universe.

True

Dark matter is about 27 per cent.

2 Dark matter emits visible light.

False

It does not.

3 The acceleration of expansion was found in 1998.