Do we know for sure Bitcoin's blockchain has never been hacked? How do we know that some mysterious organization doesn't own a majority of nodes? (honest question)
I suppose we don't. But if word ever got out that it could be under attack, the value of BTC would crash, and an emergency hard fork would occur. It would take a large investment to try and control/manipulate the hash power to do that, and so to throw away all that money to kill a blockchain that can be simply forked with a new hashing algorithm seems low probability?