Dark matter is the name scientists give to matter that appears to have mass and gravity but does not interact with electromagnetic radiation in the way ordinary matter does. We cannot see it directly. Instead, astronomers detect its gravitational influence on galaxies, galaxy clusters and the large-scale structure of the Universe.
One major clue comes from galaxies. Stars far from the centers of galaxies often orbit much faster than expected from the visible matter alone. The simplest explanation is that a large amount of additional invisible mass surrounds the galaxy in a dark matter halo.
Gravity bends the path of light. By measuring how galaxies and galaxy clusters distort the light from objects behind them, astronomers can map the distribution of unseen mass. Gravitational lensing provides some of the strongest evidence for dark matter.
Dark matter plays an important role in the formation of cosmic structure. Its gravity helped matter gather into filaments, galaxy clusters and other large-scale structures. Ordinary matter then formed galaxies within this gravitational framework.