The majority of asteroids in the Solar System are found in the main asteroid belt. This is located between the orbits of Mars and Jupiter, with the greatest concentration of asteroids between 2.12 and 3.3 AU. Although it is thought to contain millions of objects (only tens of thousands have actually been observed), if they were all gathered into a single object, it would not exceed 1,500 km in diameter.
The main asteroid belt is very sparcely populated and the distribution of asteroids within it is not uniform. In particular, the break-up of larger bodies results in families of asteroids which all share very similar orbital properties and cluster together. While at the other extreme, there are zones in which there are almost no asteroids. These are known as the Kirkwood Gaps and are the result of orbital resonances with Jupiter.
It is thought that the main asteroid belt is a leftover from the early Solar System when the strong gravitational influence of Jupiter prevented the planetesimals in this region from coalescing to form a planetary core.