Question

what might be the possible origin of the asteroids

Answer 1

Asteroids, also called minor planets, are rocky bodies orbiting the Sun. Ninety percent of them have orbits between the orbits of Mars and Jupiter, a region called the asteroid belt. The largest asteroid, Ceres, is almost 600 miles in diameter and has about one-third the volume of all other asteroids combined.

Answer 2

gravitational attraction of "lumps of matter" into bigger lumps of matter.
Or maybe a big lump of matter, already "grav aggregated" getting a bit knocked off it
????
Ultimately, though, gravitational attraction I think ?