What key ingredient in the modern condensation theory was missing or unknown in the nebula theory?

Interstellar dust

Explanation:

In modern solar system theory of condensation, interstellar dust, which was lacking in nebular theory, would be the essential component. Cosmic dust is dust that exists in outer space or that has fallen to earth, also called extra-terrestrial dust or spatial powder. The majority of the cosmic dust particle sizes range from a few molecules to 0.1 μm.