Why no one can track down the hackers who stole around $500million worth of NEM from japanese exchange. how do you keep your cryptocurrency?
You cannot compare cryptocurrencies with exchanges. Both have entirely different system of working. Cryptocurrencies can never be hacked, unless an idiot or a careless wallet owner let that happen due his own foolishness. Cryptocurrencies use blockchain technology. Every day new blocks are being added to the blockchain. So if anyone who wants to hack it, he will have to hack all the wallets on the blockchain network which is impossible to do.
This is true, you absolutely cannot trust a wallet you create Online on a centralized exchange service. You can pretty much say with 100% accuracy that if you don't posses and know the private key(s)/mneumonic seed to your wallet, then you do not own the coins that are within that wallet. When you open an account on an exchange, you might give them your info and create a user name and password, but all that doesn't matter on the blockchain...
they know your private key to
your wallet on there, so therefore
they own your coins and you can't access them without the authority of the centralized party.