What do you mean take control of it? Theoretically if someone has more than 50% of the hashing power in the network it can attack the network and double spend, but this is very unlikely because that would instantly result in a collapse of the whole network. People will lose faith on BTC and stop using it. Currently, people with hash power earn money only if the network is live and people believe in it, so they have no incentive whatsoever to do such an attack.