What is the word for "Unnecessary detailed information"?

Can you tell me what the word is for "Unnecessary detailed info on a topic which is given after the prominent parts is given"?

You could say that it is extraneous:

not pertinent; irrelevant: an extraneous remark; extraneous decoration.

If someone doesn't stay on topic, you might say their unrelated information is a digression or that they've gone off on a tangent.

If they simply go on at length, they might ramble, drivel, blather, or babble, producing some form of nonsense, perhaps claptrap or prattle.

The word you're looking for is superfluous

From the Oxford Dictionary:

superfluous - Unnecessary, especially through being more than enough:

Following on aedia λ's answer:

Digressive might be another alternative.

Merriam-Webster:

digressive: characterized by digressions

Edited to include definition based on Sven's suggestion.