I have read that the bitcoin network can not be hack, as hacking it requires all computer system upon which it runs, and these are in millions, that it's too expensive to take such adventure. Although individual wallets can be hacked if not properly secured or exposed due to compromised.
Security has been always an issue in the internet world so hacking bitcoin is not impossible. It's internet related so it's really possible to be steal from us. Well as long as we are taking care of our own account properly then we are assured of its safety. Nothing is impossible for those people who really wants to get something.