The visible universe—including Earth, the sun, other stars, and galaxies—is made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter makes up only 4.9% of the mass of the universe (heavy elements [<0.1%], neutrinos [0.3%], stars [0.5%], free hydrogen and helium [4%]).
The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (26.8%) and a force that repels gravity known as dark energy (68.3%).
Scientists have not yet observed dark matter directly. It doesn’t interact with baryonic matter and it’s completely invisible to light and other forms of electromagnetic radiation, making dark matter impossible to detect with current instruments. But scientists are confident it exists because of the gravitational effects it appears to have on galaxies and galaxy clusters.
In 1929 Edwin Hubble proved that our universe is expanding by showing that the farther a galaxy is from us, the faster it is speeding away into space. The Hubble Constant is the rate at which the universe is expanding, i.e., 67.15 km/s/Mpc, with just 1.2% uncertainty. This means that theoretically a galaxy 1 Mpc (3.26 million light-years) distant is moving away from Earth at 67.15 km/s, and a galaxy 2 Mpc (6.53 million light-years) distant is moving away at twice that speed, 134.3 km/s.
In 1998, two teams of scientists found that the universe isn’t just made of matter. It also contains some odd material – dark energy – that counteracts its gravitational pull and speeds up cosmic expansion, and the force seems to be growing stronger as the universe expands. Whatever it is, is still unknown.