
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.

subject
A particular topic, subject area, or field of knowledge that one is dealing with.
Um diese Prüfung zu bestehen, musst du tief in die Materie eintauchen.
To pass this exam, you need to dive deep into the subject.
'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
Thema usually refers to the specific subject of a conversation, text, or presentation. Materie, by contrast, refers more to an entire field of knowledge or subject area and can therefore sound broader and more formal. For a single issue being discussed, Thema is the right word; for a larger field of knowledge, Materie is more appropriate.

Nach mehreren Seminaren versteht sie die komplizierte Materie des Steuerrechts besser.
After several seminars, she understands the complicated subject matter of tax law better.
Das Thema des heutigen Vortrags ist die Einkommensteuer.
The topic of today's presentation is income tax.