After looking at the hashrate repartition https://blockchain.info/fr/pools. It seems to me that bitcoin is quite centralized, and that two big actors (mining pools company who run the bitcoind server) like BTC guild and GHash.IO suffice to obtain the mythical 51% computing power, allowing various kinds of attacks.
How do we know that these two guilds are not the same person, or will not collude? In the event that such collusion happens, and malicious double spending* occurs or other attack occurs, how do we detect it? What happens when we do detect it?
*Double spending doesn't have to be malicious: https://blockchain.info/fr/double-spends