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Wikipedia gives literal translation as: Into the middle of things.
As far as I am aware into – in takes accusative.
Plural accusative of medium seems to be media, not medias
Even if I am mistaken in this way of thinking, there seems to be no medias word in declension table for medium.
Why does in medias res seem to be accepted form then?
You are confusing two words:
In this idiom one goes into "central things". The word res is feminine (the singular nominative and plural accusative happen to look alike), so the adjective has to be in feminine plural accusative: medias.
If you were to use the noun medium instead, you might want to go into "the center of things". Then you'd put medium in singular accusative (there's only one center, right?) and res in plural genitive. This leads to the alternative in medium rerum. This might be equally valid, but Latin does often prefer adjectives to nouns, and Horatius happened to write in medias res.
Using the adjective is probably more idiomatic and common than the noun, but the noun is not a non-Latin choice either. See this question and its answer for an example from Vergilius, where the noun medium is used although the adjective medius would have been metrically equal. Perhaps the adjective would not work here, as something happens between two things, not in the middle of one. But I would still take it as evidence (perhaps weak!) that both medius and medium are appropriate in cases like yours.
Medias is not a noun but an accusative of the adjective medius (middle or central) in agreement with the plural res. This is standard idiomatic usage, not just for this phrase. A spatial portion is consistently expressed (p. 168) in Latin with an adjective in a way that sounds wrong when taken word-for-word into English:
- 'The whole of,' 'the middle of,' 'the top (bottom) of,' 'the rest of,' are expressed in Latin by adjectives agreeing with nouns (instead of the partitive genitive)
The alternative in medium rerum constructed by Joonas Ilmavirta (using the accusative noun medium and the genitive rerum) is the closest word-for-word to the English "into the middle of things", but is not "equally valid" based on the Latin usage rule.
As the Latin preposition 'in' can govern either the accusative or the ablative case, depending on whether movement or place is expressed, it is grammatically correct to speak of (for example) an author plunging in medias res (into the midst of matters) to begin the presentation of a story, but (theoretically, at any rate) incorrect to say that a story begins in medias res as this is evidently place and not movement, and strictly should be in mediis rebus, the ablative form. But most people would dismiss this as pedantry, like trying to put status quo into the ablative.
