Galaxies rushing away from each other

An expanding universe

The detection of extragalactic redshift by Vesto Slipher, Britannica says, led to the realization in the 1920s that all galaxies are receding. Britannica Edwin Hubble correlated these observations in mathematical form to provide evidence that the universe is expanding. Britannica

We know this, he says, because we see galaxies rushing away from each other, not from a central point, and because leftover heat uniformly fills the universe. NASA

In a Nobel Prize winning discovery, Hubble uncovered the acceleration of our expanding universe along with ground-based observatories. NASA For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate. NASA

Gravitational lumps eventually formed a cosmic web of filaments and vast sheets of matter that shape the underlying structure of the universe we see today. NASA

For much of the 20th century, it was an open question whether the universe in the future will continue to expand indefinitely or will eventually collapse back. Britannica The dark matter that has been deduced from various dynamic arguments is about 23 percent of the universe, and dark energy supplies the remaining amount, bringing the total average mass density up to 100 percent of the closure density. Britannica

Ordinary atoms (also called baryons) make up only about 5% of the universe, in WMAP's census. NASA Dark matter (matter not made up of atoms) is about 25.0%. NASA Dark energy, in the form of a cosmological constant, makes up about 70% of the universe, causing the expansion rate of the universe to speed up. NASA

WMAP’s measurements, in Britannica wording, showed that today the universe is 72.6 percent dark energy, 22.8 percent dark matter, and 4.6 percent atoms. Britannica