This dominance enables the attackers to double-spend coins, prevent other transactions from being confirmed, and potentially halt the mining of new blocks. By overwhelming the network with their mining power, they can override the consensus rules and execute fraudulent activities, which undermines the trust in the blockchain.
51% Attack Example
For example, in a blockchain with a small number of miners, if one miner or group of miners manages to control 51% of the total hashing power, they could spend their coins and then reverse transactions to regain those coins, effectively allowing them to spend the same coins twice.