I have no knowledge of any other cause for the angular momentum of any celestial body.

The cause is that Earth was rotating 4.5 billion years ago. What made that happen? Lots of things. The collision with Theia changed the Earth's angular momentum by quite a bit. Before that, the proto-Earth had some rotational angular momentum due to its formation from a rotating gas cloud. Before that, who knows? For example, a star passing nearby the gas cloud from which our solar system formed could have given the gas cloud some angular momentum due to gravity gradient torque.

 

Bottom line: A long time (9 billion years or so) passed between the big bang and the formation of our solar system. Lots of things could have happened in that long time to transfer angular momentum to what eventually became our Earth.

 

 

The fact that all celestial bodies are revolving & spinning suggests that it is not the result of a random event like "a star passing nearby".

The fact that all celestial bodies are revolving & spinning suggests that it is not the result of a random event like "a star passing nearby".

 

No, not really. It's not like all of the rotation is along parallel axes throughout the universe. That's not even true in our solar system.