First one seen in 1801 by Giuseppi Piazzi. Named it Ceres. The asteroid belt is mostly space and very little matter.
Spacecraft can safely fly through it without having to steer around objects.
Gravity from Jupiter keeps the asteroids stirred up and keeps them from joining together to form a new planet. Gaps, called Kirkwood Gaps, are formed in the asteroid belt by Jupiter's gravity. Kepler's 3rd law relates orbital periods with orbital size, so all asteroids with those certain orbital periods have certain orbital size. Asteroids outside the Asteroid Belt Lagrange points Two stable locations in a planet's orbit path. Some asteroids have eccentricity that brings them into the inner solar system.