Everything you can see makes up under 5 percent of the universe
Stars, planets, gas and people are all built from ordinary matter, which accounts for only about 4.84 percent of the universe's mass and energy. Invisible dark matter makes up roughly a quarter, and mysterious dark energy nearly 70 percent. Even within ordinary matter, the shining stars, planets and visible clouds amount to only about 6 percent.
The word comes via Old French from the Latin universus, meaning combined into one. Greek philosophers from Pythagoras on called it 'the all', or kosmos. Early models placed Earth at the centre; the Scientific Revolution moved the Sun there; later astronomers realised our galaxy holds a few hundred billion suns, and the observable universe holds a comparable count of galaxies. On the grandest scale galaxies are spread evenly, but closer in they cluster into filaments around vast voids, giving space a foam-like texture.
The universe's early history unfolds in steps. For roughly its first minute, energetic photons kept protons, neutrons and electrons from binding. Over the next 15 minutes or so, fusion forged helium plus traces of deuterium and lithium, and nothing heavier. At around 47,000 years, matter overtook radiation as the dominant influence. After about 377,000 years, the cooling plasma let electrons and nuclei form the first neutral atoms, the universe turned transparent, and the light released then still reaches us as the cosmic microwave background. The first stars, probably huge and short-lived, lit up 100 to 300 million years in.
About 9.8 billion years after the Big Bang, matter had thinned out enough that dark energy, whose density stays constant, took over, and expansion began accelerating. In 1998 two teams independently measured the slowdown they expected and found the opposite. The average density of mass and energy today is equivalent to roughly five protons per cubic metre, a balance that let the universe expand for 13.8 billion years rather than collapsing or flying apart too fast for galaxies to form.
Because light has a finite speed, only part of the universe is visible. The observable region's edge now lies 46 billion light-years away, making it about 93 billion light-years across, even though its light has travelled for only 13.8 billion years. Whether the whole universe is finite or infinite is unknown. For scale, the Milky Way spans roughly 87,400 light-years.
Source: Universe
