Matter and forces

The way that matter is spread across the universe is controlled by four fundamental forces, which influence different types of matter in different ways. Visible matter is composed of tiny atoms that are bound to each other by the electromagnetic force. These are built from even smaller elementary particles held together by both electromagnetism and two ‘nuclear forces’ that are much more powerful but only operate across tiny distances.

When matter accumulates in large quantities, another force, gravitation, or gravity, comes into play. Gravitation attracts objects with mass. It’s extremely weak between individual atoms, but grows stronger as they accumulate in bulk, and has an effectively limitless range. By drawing material together on a variety of scales, gravity is responsible for the formation of stars, galaxies and even galaxy clusters. However, there has not been enough time since the universe was born for it to have shaped the very largest structures in the universe: billion-light-year chains or filaments of galaxy clusters and superclusters that surround apparently empty voids. This large-scale distribution of matter originates instead from density variations that have been with the universe since birth. Such ‘ripples’ can be seen as tiny differences in the wavelengths of the CMB radiation released as the universe became transparent.