die Materie
matter
In physics, the substance that all physical objects consist of and that has mass and volume.
Das Universum besteht aus Materie, Energie, Raum und Zeit.
The universe consists of matter, energy, space, and time.

matter / subject
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matter
In physics, the substance that all physical objects consist of and that has mass and volume.
Das Universum besteht aus Materie, Energie, Raum und Zeit.
The universe consists of matter, energy, space, and time.

'Die Materie' does not mean 'problem' or 'affair' like the English word 'matter' (e.g., in 'What's the matter?'). It refers only to physical substance or to a complex subject matter.
Dunkle Materie ist eines der größten Rätsel der Physik.
Dark matter is one of the greatest mysteries of physics.
When 'Materie' (subject matter) refers to a subject or topic, it is often used in fixed expressions, e.g., 'sich in die Materie einarbeiten' (to familiarize oneself with the subject), 'tief in der Materie stecken' (to be deeply immersed in the subject), or 'die Materie beherrschen' (to master the subject matter).
Für ihre Doktorarbeit musste sie sich tief in die Materie einarbeiten.
For her doctoral thesis, she had to familiarize herself deeply with the subject.
The context makes the meaning clear: in scientific texts (physics, chemistry), it is about physical substance. In texts about learning, work, or research, it is about a field of knowledge.
Antimaterie zerstrahlt bei Kontakt mit normaler Materie.
Antimatter annihilates when it comes into contact with normal matter.
topic / theme
'das Thema' and 'die Materie' can both mean an area that you are dealing with. 'das Thema' is the normal word for the content of a conversation, text, or lesson. 'die Materie' refers more to a subject area or field of knowledge that you work your way into more deeply. It sounds a bit more formal and often more difficult or technical. The words can be confused because both say what something is about. For a conversation or presentation, 'Thema' usually fits. If you want to emphasize that a field is complex, 'Materie' fits.

Nach ein paar Wochen hat sich Mehmet gut in die neue Materie eingearbeitet.
After a few weeks, Mehmet had worked his way well into the new subject.
Im Deutschkurs ist Wohnen heute das wichtigste Thema.
In German class, housing is the most important topic today.