vague, indistinct, and poorly-defined, difficult to describe, understand, or see;
e.g. That is a nebulous explanation for what happened, and it will only be plausible if you provide more details.
(adjective)
relating to or characteristic of one of the clouds of dust and gas found in space, i.e. characteristic of a nebula, typically in terms of appearing cloudy, hazy, or unclear;
e.g. A dense, nebulous fog settled over the city, disrupting commuters.