The Oort Cloud is a vast, roughly spherical shell of icy, comet-like objects that scientists think surrounds the Sun, the planets, and the Kuiper Belt. Astronomer Jan Oort proposed it in 1950 to explain where long-period comets come from, and it has never been observed directly, so it remains a theorized region.
According to NASA, its inner edge is thought to lie about 2,000 to 5,000 astronomical units (AU) from the Sun and its outer edge somewhere between 10,000 and 100,000 AU, where 1 AU is the Earth–Sun distance of about 93 million miles. That is far beyond Pluto, which orbits at roughly 30 to 50 AU; even Voyager 1 will take about 300 years to reach the cloud’s inner edge. Its objects are believed to be leftover planetesimals that the giant planets, mainly Jupiter, flung outward after the solar system formed about 4.6 billion years ago.
Source: NASA Science, Oort Cloud Facts
Image: Oort cloud Sedna orbit.jpg by NASA/JPL-Caltech/R. Hurt, public domain, via Wikimedia Commons.