Bitcoin stolen from Electrum 2.9.3 my btc were stolen with an anonymous transaction. How was possible? Electrum should be a offline secure wallet, besides they were stolen during the night , with my desktop offline. The receiver address is visible. Can i retreive my btc? Moreover, in the future how can i trust every offline wallet? I donwloaded the electgrum sofware from the official site.
Easy.
Someone has/had remote access to your wallet/files.  They don't need access to the computer during the time of a transaction.  Example:  Send me your wallet and go ahead and secure your computer all you want.... I can simply use your wallet on my machine and spend the funds.  Also, it doesn't need to even be your wallet as I would only need your private key or your seed.   It's likely your computer is infected, they downloaded the wallet, and simply waited to withdraw.

No, you can't get your bitcoin back.  Once gone, it's gone.

The fact is, your computer wasn't always offline since you admittedly used it to download the wallet software.

What you should do?  Well, nothing about the bitcoins that are gone, but know that you should never use that seed/wallet again.  It's already compromised.
Again, what you should do?  Get a hardware wallet like Trezor and know that even if your computer is infected with a virus, your coins are still safe.  (As long as you don't backup the seed via notepad on the computer.)