Bitcoin is very secure, because it is one of the most powerful networks in cyberspace. 5 million Bitcoin miners are supported by 15 gigawatts (GW) of energy, and 15 gigawatts is more energy than the US Navy uses.
If you want to prevent me from doing transactions on the bitcoin network for 8 hours, you need to generate an additional 3,000 exahashes (EH/s) and it will cost you about 10 years and 100 billion dollars.
- Michael Saylor