Because they have been observed and space probes have navigated through them.
They are concentrated between the Mars and Jupiter.
Scientists think that the asteroid belt is a group of planetesimals (solid objects) that are caught between the gravity of Jupiter and the Sun.
I think that its asteroids
The largest collection of asteroids is found orbiting the Sun between orbits of Mars and Jupiter, This area is sometimes called the "asteroid belt". Think about it this way: the asteroid belt is a big highway in a circle around the Sun. The asteroids are like cars on the highway, except that they are thousands of miles apart on the average. Even so, sometimes the asteroid cars run into one another. When this happens, the asteroids may break up into smaller asteroids. Scientists think that most asteroids are the result of collisions between larger rocky space bodies. Asteroids can be a few feet to several hundred miles wide. The belt probably contains at least 40,000 asteroids that are more than 0.5 miles across. If an asteroid is disturbed by the gravitational pull of a planet, or is involved in a collision, it can be thrown out of the belt and go into orbit as a moon. Some of Jupiter's many small moons were likely once asteroids.
The largest collection of asteroids is found orbiting the Sun between orbits of Mars and Jupiter, This area is sometimes called the "asteroid belt". Think about it this way: the asteroid belt is a big highway in a circle around the Sun. The asteroids are like cars on the highway, except that they are thousands of miles apart on the average. Even so, sometimes the asteroid cars run into one another. When this happens, the asteroids may break up into smaller asteroids. Scientists think that most asteroids are the result of collisions between larger rocky space bodies. Asteroids can be a few feet to several hundred miles wide. The belt probably contains at least 40,000 asteroids that are more than 0.5 miles across. If an asteroid is disturbed by the gravitational pull of a planet, or is involved in a collision, it can be thrown out of the belt and go into orbit as a moon. Some of Jupiter's many small moons were likely once asteroids.
Asteroids are mainly found in the asteroid belt, which is located between the orbits of Mars and Jupiter in our Solar System. These asteroids range in size, shape, and composition and are remnants from the early formation of the Solar System.
Throughout the solar system. We think of the "asteroid belt" as a roughly toroidal (doughnut-shaped) region of space between Mars and Jupiter, but in fact there are asteroids close to Earth, in between Earth and Venus, and in closer to the Sun than Venus is. We don't have a good reckoning as to how many asteroids are out beyond Jupiter; most asteroids are too small to be seen, that far away. But we can be confident that there are quite a few! Beyond the orbit of Neptune, we no longer call them "asteroids"; they are referred to as "Trans-Neptunian Objects" or even further out, "Kuiper Belt Objects". But they are merely asteroids by another name.
One thing Galileo was famous for was studying Jupiter and its moons.
Some scientists believe Jupiter played a role in protecting Earth from frequent asteroid impacts that could have otherwise inhibited the evolution of life. Jupiter's strong gravitational force can deflect or capture incoming asteroids, reducing the number of impacts on Earth. This allowed life to flourish by providing a more stable environment for evolution.
i am not %100 sure but some think in the begining there we only 3 sons and then the big bang......ect.......then the three sons were smashed into milions of bits so small you could not see them but there were only 3 to make orions belt but the real reson is that's how God created it when i wrote sons i ment suns soz
Scientists may not have named all of the asteroids in the asteroid belt because there are so many that they are hard to keep track of.
At the same time that the Earth (and the other planets) formed : 4.5 billion years ago.