I'm building a voting Dapp using ethereum blockchain but I have a doubt. The best thing about blockchain is that it's decentralized, transparent and secure i.e. no middleman or centralized server controls it. But if it's on ethereum, isn't it controlled by ethereum? If someone hacks it or tampers it, we go back to recentralise it, isn't it?
Ethereum is a decentralized blockchain. It's not controlled by anyone and it's run by client nodes. Anyone is free to run their own node which participates in upkeeping the blockchain and nobody can be banned from connecting their node. There is no company called Ethereum.
So you can't hack Ethereum per se. In theory you can hack the protocols which are used to communicate various things around the blockchain or the actual Ethereum implementation but so far nobody has found a way to do it - at least not in any way which would be detrimental for the network as a whole.
There have been attempts to disrupt the network and one attempt even managed to hard fork the network - The DAO hack in 2016 ( https://medium.com/swlh/the-story-of-the-dao-its-history-and-consequences-71e6a8a551ee ). But even if the network/blockchain is sometimes disrupted it never turns into a centralized blockchain.