Stars, planets, nebulae, galaxies and all other observable ordinary matter account for only about 4.9-5% of the universe’s contents, according to recent cosmological measurements, while dark matter makes up about 27% and dark energy roughly 68%. Although these two components dominate about 95% of the universe, their nature remains unknown, and scientists infer their existence from their effects on the motion of galaxies and the expansion of space.
Contrasting effects on the evolution of the universe
Dark matter gives galaxies and galaxy clusters additional gravity that helps hold them together, while dark energy is linked to the accelerating expansion of space and the increasing separation of galaxies. Observations from ground- and space-based observatories, including the European Space Agency’s Euclid mission, support this view of the composition of the universe.
The relationship between the components of the universe appears to involve two opposing effects: the gravity of ordinary matter and dark matter slows the expansion, while dark energy drives the universe to expand at an accelerating rate.
How did evidence of dark matter emerge?
Early indications of the existence of invisible mass emerged at the beginning of the 20th century, when astronomers measured the movement of stars within galaxies and found that their rotational speeds were higher than could be explained by their visible mass. In the 1930s, Swiss astronomer Fritz Zwicky provided the first evidence of missing mass while studying the movement of galaxies in the Coma Cluster.
In the 1970s, American astronomer Vera Rubin strengthened this idea after showing that stars at the edges of galaxies rotate at speeds close to those of stars near their centers, contrary to what would be expected from the distribution of visible matter alone.
Dark matter does not emit, absorb or reflect light, so conventional telescopes cannot detect it directly. But its gravitational effect is visible in the rotational speeds of galaxies and in gravitational lensing, in which light from distant galaxies bends as it passes near massive, invisible objects.
Cosmologists believe dark matter served as a “cosmic scaffolding,” with its gravity helping, over billions of years, to gather ordinary matter in the regions where stars, galaxies and clusters formed. Its nature, however, remains unknown. Theories suggest it may consist of undiscovered subatomic particles, such as weakly interacting massive particles or axions, while scientists rule out the possibility that it is ordinary matter, antimatter or black holes.
According to current scientific theories, billions of dark matter particles may pass through human bodies every second without anyone noticing, because they rarely interact with ordinary matter.
Dark energy and the accelerating expansion of space
Dark energy’s effect is the opposite of dark matter’s, as it is associated with driving space toward accelerated expansion. Its effects emerged in 1998, when two international teams studied explosions of distant supernovae and discovered that the expansion of the universe was accelerating rather than slowing under the force of gravity, as had been expected.
The explanation for dark energy remains under study. Leading hypotheses include the possibility that it is energy inherent in the vacuum itself, a changing cosmic field, or that the current understanding of gravity needs to be revised.
We know how dark energy behaves, but we do not know what it is.
Mapping galaxies in search of answers
The Dark Energy Spectroscopic Instrument (DESI) project in the United States and Europe’s Euclid mission are mapping millions of galaxies to investigate whether the properties of dark energy remain constant over time or change. The answer could affect scientists’ understanding of the universe’s history and future.
The European Space Agency, or ESA, is participating in this research through the Euclid mission, along with Fermilab, the DESI project partnership and other educational and research programs. NASA and the European Space Agency are also continuing to develop more precise tools for mapping dark matter and dark energy.
Nearly a century after the first evidence of dark matter emerged, the explanation of roughly 95% of the universe’s contents remains in its early stages, meaning that any new discovery could potentially reshape prevailing ideas about the origin, evolution and fate of the universe.
