Oort Cloud:

The Oort Cloud is an immense spherical cloud surrounding our Solar System. Extending about 30 trillion kilometers (18 trillion miles) from the Sun, it was first proposed in 1950 by Dutch astronomer Jan Oort. The vast distance of the Oort cloud is considered to be the outer edge of the Solar System - where the Sun's orb of physical and gravitational influence ends. 

The Oort Cloud contains billions of icy bodies in solar orbit. Occasionally, passing stars disturb the orbit of one of these bodies, causing it to come streaking into the inner solar system as a long-period comet. These comets have very large orbits and are observed in the inner solar system only once. In contrast, short-period comets take less than 200 years to orbit the Sun and they travel along the plane in which most of the planets orbit. They come from a region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper, who proposed its existence in 1951.

Within the cloud, comets are typically tens of millions of kilometers apart. They are weakly bound to the sun, so passing stars and other forces can readily change their orbits, sending them into the inner solar system or out to interstellar space. This is especially true of comets on the outer edges of the Oort cloud.

Tidal and molecular forces also contribute to influencing the orbits of bodies within the Oort Cloud. A giant molecular cloud is by far more massive than the Sun. It is an accumulation of cold hydrogen that is the birthplace of stars and solar systems. These are infrequently encountered, about every 300-500 million years, but when they are encountered, they can violently redistribute comets within the Oort cloud. The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth. This matter comes from different places in the Solar System, and from different distances from the Sun - this explains the varying chemical compositions among these comets.