The Oort Cloud
The Oort Cloud is a theorized shell of icy objects that lie beyond the Kuiper Belt. It lies in the outermost reaches of our solar system. It is named after astronomer Jan Oort, who first theorized its existence.
The Oort Cloud is roughly spherical, and is thought to be the origin of most of the long-period comets that have been observed.
This cloud of particles is theorized to be the remains of the disc of material that formed the Sun and planets. Astronomers now refer to those primeval objects as a protoplanetary disk.
The most likely theory is that the material now in the Oort Cloud probably formed closer to the young Sun in the earliest epochs of solar system formation. As the planets grew, and in particular as Jupiter coalesced and migrated to its present position, its gravitational influence is thought to have scattered many icy objects out to their present position in the Oort cloud.
The Oort Cloud is very distant from the Sun, the inner limits of the Oort Cloud begin at about 2,000 AU from the Sun and it can be disrupted by the nearby passage of a star, nebula, or by actions in the disk of the Milky Way. Those actions knock cometary nuclei out of their orbits, and send them on a headlong rush toward the Sun.
The Oort Cloud is a reserve of cometary nuclei that contain ices dating back to the origin of the solar system. No one knows for sure how many objects exist in the Oort Cloud, but most estimates put it at around 2 trillion.
The existence of Oort cloud is purely hypothetical, it is not visible over a telescope for observation as each object in it is set apart nearly as the distant as earth is from Saturn.
http://space-facts.com/oort-cloud/