What causes some scientists to be so sure that Oort belt exists when there is no direct evidence of the Oort belt of comets?
The Oort cloud explains the existence of long period comets.
Long period comets with periods of over 200 years have to come from somewhere. One problem with such comets is that they will eventually either fall into the Sun or be expelled from the solar system.
The comets had to have been around from the early solar system, so they must have been somewhere for a long time before becoming a comet.
The Oort cloud explains this. The comets would have been in the Oort until some gravitational perturbation caused them to fall into cometary orbits.
Also, models of the early solar system predict that the large outer planets, particularly Jupiter, would have ejected material as they moved to their current positions.
The problem with actually detecting the Oort cloud is that it is between 2,000 and 50,000 AU away. It is possible that modern space telescopes, such as Kepler, may be able to detect Oort cloud objects transiting a star.