However, if some company actually gets a share of 50% of the mining market by buying big mining companies like Bitmain, private companies,... Or if the major pools decide to work together with other major pools and get more than 50% of the hash rate. They would be able to easily manipulate the blockchain,because almost every second block is mined by them.
>50% attack
Also referred to as a majority attack. If the attacker controls more than half of the network hashrate, the previous attack has a probability of 100% to succeed. Since the attacker can generate blocks faster than the rest of the network, he can simply persevere with his private fork until it becomes longer than the branch built by the honest network, from whatever disadvantage.
No amount of confirmations can prevent this attack; however, waiting for confirmations does increase the aggregate resource cost of performing the attack, which could make it unprofitable or delay it long enough for the circumstances to change or slower-acting synchronization methods to kick in.
Source:https://en.bitcoin.it/wiki/Double-spending