existierend
existing
Actually present, existing in reality.
Die Studie basiert auf allen existierenden Daten zu diesem Thema.
The study is based on all existing data on this topic.

existing
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existing
Actually present, existing in reality.
Die Studie basiert auf allen existierenden Daten zu diesem Thema.
The study is based on all existing data on this topic.

'Existierend' (existing) is the present participle of the verb 'existieren' (to exist). Like many present participle forms in German, it can be used as an adjective before a noun.
Alle existierenden Dokumente müssen geprüft werden.
All existing documents must be checked.
This adjective is used almost exclusively before a noun (attributively). Predicative use after verbs like 'sein' (to be) is very uncommon and sounds unnatural.
Die existierenden Regeln sind für alle verbindlich.
The existing rules are binding for everyone.
The word 'existierend' (existing) belongs to formal written language, technical language, or administrative language. It is rarely used in normal everyday conversation.
Der Plan muss an die bereits existierende Infrastruktur angepasst werden.
The plan must be adapted to the already existing infrastructure.
really
'wirklich' and 'existierend' can both say that something is not just imagined or invented. However, 'wirklich' is much more general and very natural in everyday language. It can also be used to emphasize a statement. 'existierend' sounds more precise and somewhat more factual. It fits especially when you want to say that something is actually present, for example a file, a building, or a problem. Put simply: 'wirklich' often fits in normal conversation, while 'existierend' is more likely when it is about the existence or presence of something.

Bitte lösche nur alte Kopien und keine existierenden Kundendaten.
Please delete only old copies and no existing customer data.
Das ist keine Ausrede, das ist ein wirkliches Problem.
That is not an excuse; it is a real problem.