The Local Group
The Milky Way, Andromeda and the Triangulum Galaxy are the largest members of the Local Group. Dozens of smaller dwarf galaxies orbit them or move through the same gravitational region. The group extends over several million light-years.
Dwarf galaxies are difficult to detect because they contain few stars and have low surface brightness. Some are dominated by dark matter, while others have lost much of their gas after passing through the halos of the large spirals.
From groups to clusters
A galaxy group is a relatively small gravitational association. A galaxy cluster is larger and can contain hundreds or thousands of galaxies, together with enormous quantities of hot plasma. The space between galaxies is not empty: it contains gas heated to tens of millions of degrees.
Clusters grow by merging with smaller groups and by accreting galaxies along cosmic filaments. Their mass can be measured from galaxy motions, X-ray gas and gravitational lensing.
Dark matter and gravitational lensing
Most of a cluster’s mass is dark matter. Its gravity bends the light of background galaxies, creating arcs, multiple images and magnified views of the distant universe. This effect is called gravitational lensing.
Lensing maps reveal the invisible mass distribution and can show where dark matter is displaced from hot gas during a cluster merger. These observations test models of structure formation and the nature of dark matter.
Hot gas between the galaxies
The intracluster medium is so hot that it shines mainly in X-rays. It is enriched with elements made in stars and supernovae. The gas records the history of star formation and the energy released by active black holes.
Although the gas is thin by terrestrial standards, it contains more normal matter than the stars in many clusters. Understanding its temperature and pressure is essential for estimating cluster masses.
A map of cosmic history
Large surveys identify clusters across different distances. Nearby clusters show mature systems, while distant clusters are seen at younger ages because their light has travelled for billions of years.
Groups and clusters connect individual galaxies to the cosmic web. They show how gravity amplifies small differences in the early universe until matter gathers into filaments, walls, groups and enormous empty regions.
Clusters as distance tools
Clusters can also help establish distances and test cosmological models. Their member galaxies share a broad redshift and their hot gas has a measurable temperature and pressure. Lensing adds an independent estimate of the total mass. When these measurements agree, a cluster becomes a powerful cross-check on the relationship between matter, geometry and cosmic expansion. When they disagree, the difference can reveal missing physics or an incomplete model of the gas.