Question: I have heard a Muslim speaker say that the Qur’an contains guidance on everything. He was speaking with reference to the following verse: 

We did not leave anything out of this Book, then all will be gathered before their Lord [for judgement]. (6:38) 

    Answer: Whether the Qur’an contains guidance on everything or not is another issue. However this much is certain that this verse cannot be presented in support of this popular view. The context of the verse shows that the verse has a specific connotation. I’ll try to explain it to you briefly. 
    6:37 says that the disbelievers demand that they be shown some sign that they profess belief. It is evident from later verses that the word ‘sign’ actually refers to the punishment the disbelievers were threatened with by the Prophet (sws) if they rejected him. 

Say: ‘What do you think, if there come upon you the punishment of God, or the Hour [that you dread], would you then call upon other than God? -- [Reply] if you are truthful! ‘Nay, -- On Him would you call, and if it be His Will, He would remove [the distress] which occasioned your call upon Him, and you would forget [the false gods] which you join with Him!’ (6:40-41)

    Consequently, the disbelievers have been quoted by the Qur’an at many instances saying that they would like to see the punishment they are being threatened with in order to see whether Muhammad (sws) was a true messenger of God. At all such places, they are answered that if this sign is shown to them, then they would not be given any further respite -- they would be destroyed. So it is better that instead of demanding this ultimate sign, they pay heed to the numerous other signs found in abundance around them and within their own being. 
    This is precisely what has been stated  in 6:37 and in the earlier part of 6:38: 

They say: ‘Why is not a Sign sent down to him from his Lord?’ Say: ‘God has certainly power to send down a Sign: but most of them understand not. There is not an animal [that lives] on the earth, nor a being that flies on its wings, but [forms part of] communities like you.  (6:37-8)

    The disbelievers are told that God has all the power to send down such a sign, but most of them do not know its implications. For when such a sign is sent, it destroys the people. So instead of demanding such a sign, they should look around and they will find plenty of signs. If they contemplate even on the animals around them and on the birds above them they will find many lessons. They will find in the individual and collective lives of these species the manifestations of the Almighty’s mercy, power, providence and wisdom. These manifestations show that this world has been made for a specific purpose by the Almighty.
    In other words the expression: ‘We did not leave anything out of this book’ if taken in context means that as far as signs to profess belief are concerned, this Book has plenty and that nothing has been left out of it. The verse does not imply that the Qur’an contains guidance on everything.