What if a malicious owns more than half of the nodes in bitcoin network? my question is simply What if a malicious owns more than half of the nodes in bitcoin network? she can simply fraud then. isn't there any algorithm to prevent someone from having more than half of nodes in bitcoin network?
No. The 51% hack is considered one of the main ways a hacker could control the network. However, it is almost completely implausible because of the sheer amount of power that would be required to have 51%. For example, the total hash rate of the network is currently 6.09 Exahashes per second. So, if a hacker wanted to take control of the network, he would need to buy about 6.1 Exahashes per second of hash power. An antminer S9, for 3,000 dollars, gives you 14 terrahashes per second. Thus, the supposed hacker would need to buy 435,000 Antminer S9's which would cost 1.305 billion. That's not even considering the growth rate of the network and the sheer amount of space and energy required to host all of those Antminers.