The universe is made up of 68.3% "dark energy" (an unknown substance that causes the universe's expansion to accelerate), 26.8% dark matter (an unknown form of matter) and 4.9% ordinary atoms, very precisely measured from the cosmic microwave background — the afterglow of radiation from the Big Bang.
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Image credit: X-ray: NASA/CXC/SAO; Infrared: (Herschel) ESA/NASA/Caltech, (Spitzer) NASA/JPL/Caltech, (WISE) NASA/JPL/Caltech; Infrared: NASA/ESA/CSA/STScI/Webb ERO Production Team; Image processing: NASA/CXC/SAO/J. Major, CC BY