[lekevicius]

I'm curious about this question, even though it's a bit off-topic: how likely is it that an entity (commercial or governmental) could "secretly" produce enough mining hardware to consistently have 51% of network's hash power?

[ntoskrnl]

The biggest mining pools have known identities[1]. If some unknown group started mining and the hashrate jumped 50%, it would draw lots of attention. There's no way it could be done secretly. A more likely attack would be a government bribing or coercing a few of the biggest mining pools to manipulate their blocks.

[1]: https://btc.com/stats/pool

[IncRnd]

How unlikely is it for that not to have already happened?