Learn French: Ce que j'ai vu, c'est une métamorphose en l'espace d'une semaine. - What I saw is a metamorphosis within the space of a week.

While en une semaine (in a week) would be grammatically correct, en l'espace de (in the space of / in the span of) adds emphasis. It highlights how remarkable or surprising it is that such a massive change (une métamorphose) happened within such a short, confined timeframe.

Could I say dans l'espace de since dans also means 'in'?

No, en l'espace de is a fixed idiomatic expression for time. Even though dans is often used for physical spaces or future timeframes, this specific phrase for duration always requires en.

Why is there a comma and c'est in the middle? Couldn't we just say Ce que j'ai vu est une métamorphose?

While omitting c'est is sometimes done in formal writing, it sounds unnatural in everyday French. When you start a sentence with a phrase like Ce que... or Ce qui... (meaning 'What...'), it is standard to 'resume' the subject with a comma and c'est. It gives the sentence a natural rhythm and adds emphasis to what follows.

As a quick refresher, why does this sentence start with Ce que and not Ce qui?