Nothing is hack-proof but BTC is so hack-resistant that it can be considered hack-proof for daily purposes. There of course is always a chance that new technology will scale computing power in one region so much that 51% will be possible (for example new type of computer that can main much faster but only one company has them)
51% attack is a real threat, I remember the moment when one pool had over 50% hash power of the whole but they were really careful about it, denied to accept new miners at that point