The Kuiper belt, named after Gerard Kuiper is a belt that starts to extend outwards of our solar system, starting at Neptune, and scientists are still not 100% sure about how big it is, although they estimate it to be 20-200 times bigger than the asteroid belt.
Similar to the Asteroid belt it contains many small (relatively) objects, mostly asteroids that just stay there, but are made of Methane, Ammonia, and Water.
The Kuiper belt also contains dwarf planets such as Pluto, Haumea, and Makemake. Some moons like Triton have thought to have originated there too. Although it is extremely huge, it is mostly stable and does not shoot out many asteroids and meteors to the inner solar system.
This is not to be confused with the Oort cloud, which is still a hypothesis. The Oort cloud expands out far after the Helioshpere ( the near middle of the Kuiper belt) and is in space outside our Solar system like a protective ball, but not super protective as it is said to send in meteorites and asteroids into the Helioshpere.
Although its origins are still unclear researchers are still working on developing hypothesis and testing them and using other methods to try to find out how it was made. Most people say that is was made of the remnants(remains) of the solar system after it had formed.
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