Is it true that Allah created the universe in 6 days?
Yes, Allaah created the heavens and the earth and everything in between in six days, as He says (interpretation of the meaning):
“And indeed We created the heavens and the earth and all between them in six Days and nothing of fatigue touched Us”[Qaaf 50:38]
This indicates that what the Jews, upon whom be the curses of Allaah, say is false, as they said that He got tired when He created the heavens and the earth in six days, so He rested on the Sabbath. Exalted be Allaah far above what they say.
More details are narrated in the Qur’aan. Allaah says (interpretation of the meaning):
“Say (O Muhammad): ‘Do you verily disbelieve in Him Who created the earth in two Days? And you set up rivals (in worship) with Him? That is the Lord of the ‘Aalameen (mankind, jinn and all that exists).
He placed therein (i.e. the earth) firm mountains from above it, and He blessed it, and measured therein its sustenance (for its dwellers) in four Days equal (i.e. all these four ‘days’ were equal in the length of time) for all those who ask (about its creation).
Then He rose over (Istawa) towards the heaven when it was smoke, and said to it and to the earth: ‘Come both of you willingly or unwillingly.’ They both said: ‘We come willingly.’
Then He completed and finished from their creation (as) seven heavens in two Days and He made in each heaven its affair. And We adorned the nearest (lowest) heaven with lamps (stars) to be an adornment as well as to guard (from the devils by using them as missiles against the devils). Such is the Decree of Him, the All-Mighty, the All-Knower”[Fussilat 41:9-12]
And Allaah knows best.