What is the nebular hypothesis?
The nebular hypothesis was proposed by the philosopher Emanuel Kant to explain how the present solar system could have come about.
Emanuel Kant visualized a spinning dust cloud or nebula that coalesced into the planets and sun of the solar system.
This is a hypothesis because there is no evidence to make up the theory. The idea that the dust cloud existed and formed the solar system was an attempt to explain the origin of the solar system by natural causes.
The empirical evidence is against the nebula hypothesis. The planets have only 1 % of the mass of solar system. If the spinning dust cloud idea was correct the planets should have 1% of the rotational momentum of the solar system. When modern astronomy allowed measurements of the rotational momentum the planets were found to have 99% of the rotational momentum.
Good theories make good predictions or hypothesis. Bad theories make predictions that don't turn out to be true, hypothesis that are proven false.