Dark Matter and Dark Energy: The Invisible Parts of the Universe

Ordinary matter, the atoms that make stars, planets, and people, makes up only about 5 percent of the universe's total energy content. Dark matter accounts for about 27 percent, and dark energy for about 68 percent. Neither emits, absorbs, or reflects light in a way telescopes can detect directly, so astronomers infer both from their effects on gravity and expansion.

What "dark" means in cosmology

In this context, dark means the substance does not interact with light in a detectable way. It does not mean absence. Dark matter pulls on ordinary matter through gravity. Dark energy changes how space expands. Both effects are measurable, even though the substances themselves remain unidentified.

Astronomers use the term dark matter for matter that gravitates but does not emit light, and dark energy for the component that drives accelerating expansion. The two are unrelated in composition, even though their names suggest a link.

The budget of the universe

Measurements of the cosmic microwave background and galaxy surveys give a consistent energy budget. The values below are rounded from recent analyses of the microwave background.

Component	Share of total energy	Main evidence
Ordinary matter	About 5%	Stars, gas, and light elements
Dark matter	About 27%	Galaxy rotation and lensing
Dark energy	About 68%	Accelerating expansion

The table shows why the invisible components matter. The familiar material of the universe is a small minority of what exists.

Dark matter: evidence from galaxies

An early hint came from Fritz Zwicky's 1933 study of the Coma cluster, where galaxies moved faster than their visible mass could hold them. Stronger evidence came from the motions of stars in galaxies. Stars orbit the center of a spiral galaxy at speeds set by the mass inside their orbit. If visible matter were the only source of gravity, stars far from the center should orbit more slowly than stars near it.

Observations instead show that rotation speeds stay roughly flat out to large distances. Astronomers, most notably Vera Rubin in the 1970s, found that this pattern requires much more mass than shines. That extra mass is spread through a broad halo around each galaxy.

Gravitational lensing as a second test

Massive objects bend light passing near them. Astronomers use this lensing effect to map mass without relying on light emitted by the mass itself. In galaxy clusters, lensing reveals far more mass than the visible galaxies and hot gas account for.

The Bullet Cluster, observed in 2006, provides a striking case. Two clusters collided, and their hot gas, which dominates the ordinary matter, slowed and piled up in the middle. Lensing mapped the mass separately, and it sits mostly where the galaxies are, not where the gas is. This separation points to a mass component that does not interact strongly with ordinary matter.

Dark matter: evidence from cosmology

The microwave background adds independent evidence. The pattern of peaks in its power spectrum depends on the amount of matter that does not interact with light. Fitting the pattern requires dark matter at about the level listed above, and the fit works with a single consistent model.

Dark matter also helps explain how structure formed. Gravity needs a mass scaffold to pull ordinary gas into galaxies in the time available since the Big Bang. Models that include cold dark matter, meaning slow-moving particles, reproduce the observed web of galaxies and clusters well.

Candidate particles for dark matter

Physicists have proposed several candidates. Weakly interacting massive particles, often called WIMPs, interact through gravity and the weak force. Axions are very light hypothetical particles that were proposed to solve an unrelated problem in particle physics. Sterile neutrinos are hypothetical neutrinos that interact even more weakly than known ones.

None of these candidates has been confirmed. Underground detectors have searched for WIMP interactions with ordinary matter, and axion experiments have searched for signals in strong magnetic fields. As of the published literature through 2024, no confirmed detection had been reported. Each null result narrows the range of allowed properties.

Dark energy: the accelerating universe

In 1998, two teams studying distant Type Ia supernovae found that these explosions appeared fainter than expected. Their light had been stretched further than a decelerating universe would allow. The simplest explanation was that the expansion of the universe is speeding up. The discovery earned the 2011 Nobel Prize in Physics.

Dark energy is the name given to whatever drives this acceleration. The simplest model treats it as a cosmological constant, a form of energy that is built into space itself. As space expands, the amount of this energy per unit volume stays roughly constant, so its influence grows as the universe grows.

Why it is strange

Vacuum energy predicted by quantum field theory is many orders of magnitude larger than the value observed for dark energy. This mismatch is one of the largest unsolved problems in physics. Some researchers propose that dark energy evolves over time, which would require new physics, but current measurements do not require that.

Dark matter versus dark energy

The two components behave very differently, and confusing them is a common mistake.

Property	Dark matter	Dark energy
Main effect	Pulls matter together	Pushes space apart
Density as space expands	Dilutes with volume	Stays roughly constant
Role in history	Seeds galaxies and clusters	Drives recent acceleration
Main evidence	Rotation curves, lensing	Distant supernovae, CMB
Identity	Unknown particle	Unknown, often modeled as a constant

The table shows that the two components dominate at different stages. Dark matter shaped the early structure of the universe. Dark energy has taken over the energy budget in the recent past.

Future mapping efforts

Several large surveys are designed to map both components. Weak lensing, which measures the faint distortion of background galaxies, traces dark matter across large areas of sky. Galaxy surveys that map the distribution of galaxies across cosmic time trace how the expansion rate has changed.

Euclid, a European Space Agency mission launched in 2023, is designed for both measurements. The Vera C. Rubin Observatory and the planned Nancy Grace Roman Space Telescope target related goals. These are described here as planned or ongoing programs, and their results will refine the current numbers rather than settle the underlying physics on their own.

Why the two numbers are trusted

The percentages above come from more than one method. Galaxy rotation, lensing, and cluster dynamics give the dark matter share. Supernova distances, the microwave background, and galaxy clustering give the dark energy share. Each method has its own systematic uncertainties, and the values agree within those uncertainties.

This cross-checking is why the standard model is treated as robust, even though the identities of the two components remain unknown. A single surprising measurement could shift one of the numbers, but a shift large enough to remove the need for either component would conflict with several independent results at once.

Key takeaways

Dark matter is inferred from the gravity it exerts on stars, gas, and light. It makes up about 27 percent of the universe's energy content, and no candidate particle has been confirmed.

Dark energy is inferred from the accelerating expansion of the universe. It makes up about 68 percent of the energy content, and its physical nature is unknown. The simplest model treats it as a constant energy of space.

Together, these two invisible components account for about 95 percent of the universe's energy. Solving their identities is among the largest open problems in physics, and the next generation of surveys is designed to test the leading ideas.