Ann: Update on Synstream Energy Corp. Heads of Agreement, page-34

@YStrategy, to answer your question in short....

Unfortunately for ambitious hackers, blockchains are decentralized and distributed across peer-to-peer networks that are continually updated and kept in sync.

Because they aren’t contained in a central location, blockchains don’t have a single point of failure and cannot be changed from a single computer. It would require massive amounts of computing power to access every instance (or at least a 51 percent majority) of a certain blockchain and alter them all at the same time.

There has been some debate about whether this means smaller blockchain networks could be vulnerable to attack, but a verdict hasn’t been reached. In any case, the bigger your network is, the more tamper-resistant your blockchain will be.