die Nebensache
side issue
A matter, detail, or aspect that is unimportant or of lesser significance compared with other things.
Das Geld ist bei dieser Entscheidung nur eine Nebensache.
Money is only a side issue in this decision.

side issue
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side issue
A matter, detail, or aspect that is unimportant or of lesser significance compared with other things.
Das Geld ist bei dieser Entscheidung nur eine Nebensache.
Money is only a side issue in this decision.

The word is made up of the preposition 'neben' (beside/next to) and the noun 'die Sache' (thing/matter). So it literally describes a 'thing that lies beside it' and is therefore less important.
Für ihn ist Pünktlichkeit die Hauptsache, alles andere ist Nebensache.
For him, punctuality is the main thing; everything else is a side issue.
'Nebensache' (side issue) is very often used as a predicate noun with the verbs 'sein' (to be) or 'werden' (to become). The pattern is: [something] is/becomes (only) a side issue.
Als sie die gute Nachricht hörte, wurden alle ihre Sorgen zur Nebensache.
When she heard the good news, all her worries became a side issue.
The expressions 'zur Nebensache werden' (to become a side issue) or 'zur Nebensache machen' (to make something a side issue) are very common. They express that something loses importance because something else comes to the foreground.
Angesichts der ernsten Lage wurde der Streit über Kleinigkeiten schnell zur Nebensache.
Given the serious situation, the argument about small things quickly became a side issue.
detail
'das Detail' and 'die Nebensache' can both refer to something small or to one aspect within a larger situation. The important difference is the evaluation. A 'Detail' is simply an individual point; it can be unimportant, but it can also be very important. 'Nebensache', on the other hand, means that something is less important compared with other things. That is why you cannot automatically call every detail a 'Nebensache'. Learners may confuse the words because small details often seem unimportant. If you just mean a precise part, use 'Detail'. If you want to say that something is not so important, 'Nebensache' fits.

Ob der Raum schön aussieht, ist heute Nebensache; wichtig ist die Technik für die Präsentation.
Whether the room looks nice is a side issue today; what matters is the technology for the presentation.
Beim Vertrag sollten wir jedes Detail genau lesen.
We should read every detail of the contract carefully.