Do we know who runs the big mining pools? 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
This list is not exhaustive, and the information is based on bitcointalk and coindesk discussion and articles. As of 1 Dec 2014, based on pool stats from few websites including https://blockchain.info/pools

GHASH.io is at 13 % , it is based out of Europe.
GHASH is closely related to CEX.io BitFury, Jeffrey Smith is the technical person at CEX.io.
Eligius 4 % is operated by Luke.Jr 
Discus Fish 25%  is based out of china &amp; coowned by Wang Chun.
Megabig power 2% in Washington operated by Dave Carlson.
KNC 8% is based out of Sweden , Sam Cole &amp; Marcus Erlandsson.
Cointerra 2%  is based out of Texas. Timo Hanke &amp; Ravi Iyengar.