The Universe is huge. Because it is expanding and that expansion is accelerating, estimates have it stretching up to 93 billion light-years across, though the visible universe is only a mere 13.8 billion light-years across. This means most of the Universe will remain invisible, as its distant light has not had enough time to reach Earth. The Universe has very low density precisely because of its size. On average, a cubic meter of space contains only 5.9 atoms. The density of matter in the Universe, however, is only one atom per every four cubic meters of space. Matter itself is mostly empty space too; a typical atom is 100,000 times larger than its nucleus, and the nucleus contains 99.9 percent of an atom’s mass.
Researchers now believe they have found more than half of the Universe’s missing baryonic matter.