Dark matter and dark energy

6 min read

Key facts

Normal matter	About 5% of the universe's total
Dark matter	About 27%
Dark energy	About 68%
First hint of dark matter	1933, Fritz Zwicky and the Coma Cluster
Galaxy rotation evidence	1970s, Vera Rubin and Kent Ford
Accelerating expansion discovered	1998, using Type Ia supernovae
Key mission	ESA's Euclid, launched July 2023
Status as of 2026	Neither has been directly detected or explained

In short

Everything we can see, from stars to gas to people, is only about 5% of the universe.
Dark matter is invisible material whose gravity holds galaxies and clusters together; it makes up about 27%.
Dark energy is an unknown effect that is making the universe's expansion speed up; it makes up about 68%.
The evidence comes from how galaxies spin, how gravity bends light, and how distant supernovae look.
Missions such as ESA's Euclid are mapping billions of galaxies to test what these two mysteries really are.

The universe we cannot see

When astronomers add up all the stars, gas and dust they can observe, the total falls far short of what gravity tells them must be there. And the way the whole universe is growing does not match what ordinary matter alone would do.

Two names cover these gaps. Dark matter is extra mass that pulls but does not shine. Dark energy is whatever is pushing the universe to expand faster over time.

"Dark" here simply means unknown and invisible to telescopes. Neither one is the same as the dark clouds of dust you can see in photos of the Milky Way.

Evidence for dark matter

In 1933, Fritz Zwicky measured galaxies in the Coma Cluster and found them moving so fast that the visible matter could not keep them together. In the 1970s, Vera Rubin and Kent Ford measured how stars orbit in spiral galaxies. Stars far from the center moved just as fast as those closer in, which only made sense if each galaxy sat inside a large, unseen halo of mass.

Gravity also bends light, an effect called gravitational lensing. Massive galaxy clusters distort the images of galaxies behind them into arcs, and the amount of bending reveals far more mass than the visible matter can supply.

The Bullet Cluster, studied in detail in 2006, is a pair of galaxy clusters that crashed through each other. X-ray images show the hot gas, which holds most of the normal matter, slowed down in the middle, while lensing shows most of the mass sailed ahead. That separation is hard to explain without dark matter.

What could dark matter be?

Dark matter does not seem to emit, absorb or reflect light, yet it has gravity. Most scientists think it is made of particles that have not yet been identified.

Popular candidates include WIMPs, heavy particles that barely interact with anything, and axions, extremely light particles. Detectors deep underground, at particle colliders and in space have searched for decades without a confirmed signal as of 2026.

A smaller group of researchers explores modified gravity instead, the idea that gravity behaves differently on galaxy scales. So far, though, no such theory explains all the evidence as well as dark matter does, especially cases like the Bullet Cluster.

Dark energy and the speeding-up universe

Since the 1920s, astronomers have known that the universe is expanding. Most expected gravity to slow that expansion down over time.

In 1998, two teams used Type Ia supernovae, exploding white dwarfs with a predictable brightness, to measure distances to faraway galaxies. The distant supernovae looked fainter than expected, meaning the expansion has been speeding up for the last several billion years. The discovery won the 2011 Nobel Prize in Physics.

The simplest explanation is a steady energy built into space itself, close to the "cosmological constant" Einstein once proposed. Recent surveys, such as DESI, have hinted that dark energy might change over time, but as of 2026 this is still being tested.

Adding it up: 5%, 27%, 68%

Combining measurements of the cosmic microwave background, the faint afterglow of the Big Bang, with galaxy and supernova surveys gives a remarkably consistent budget. Normal matter is about 5% of the total, dark matter about 27%, and dark energy about 68%.

These shares are approximate, and they describe the universe today. In the early universe, dark energy played a much smaller role, and matter dominated.

Put another way, about 95% of the cosmos is made of things we can describe by their effects but cannot yet name.

Euclid and other missions

ESA's Euclid space telescope launched in July 2023, with NASA contributing its infrared detectors. From a position about 1 million miles from Earth, it is mapping the shapes and distances of billions of galaxies across about a third of the sky.

Tiny distortions in galaxy shapes trace the dark matter between us and them, and the pattern of galaxies over time records how dark energy has stretched space. Its first images came out in 2023, and its first survey data in 2025.

Other projects add to the picture, including the Vera C. Rubin Observatory in Chile and NASA's Nancy Grace Roman Space Telescope, due to launch around 2027.

Open questions

What particle, if any, is dark matter? Is dark energy truly constant, or does it change? Could a deeper theory of gravity tie the two together?

Answering any one of these would likely reshape physics. For now, dark matter and dark energy are best seen as precise labels for real, measured effects whose causes remain unknown.