Correct.
Now does someone know if hackers can get a computer getting a public address and test it to see if the account is at 0 and then try another adress, etc...
Yes, they can. They would be wasting their time and money, but if they want to spend their life on earth walking along side Sisyphus, they are welcome to do so.
until one is not a 0 credit and then steal the money since he/she got already the private key associated ?
That will not happen as long as the addresses are generated correctly.
A hacker could test millions/billons of addresses like this
Yes they could.
and once in a while get an address with an account not at 0, is that correct?
No, that is not correct.
I am very concern about the security of my bitcoins and want to do a paper wallet after generating an address from a new computer offline, but this potential hacking method is a big concern to me.
Then you don't understand how or why bitcoin works.
Does someone has already being able to get one existing account not empty using this approach?
Anyone knows if it has already been done over the last 9 years ?
There were some very dumb people that did not generate their addresses correctly. There was also some software with bugs that did not generate addresses correctly. The bitcoins that were stored in those addresses that were not generated correctly resided in too small of a search space, and hackers were able to steal some. All addresses that are generated correctly have been secure since the beginning of bitcoin in 2009 and will remain secure for as long as our solar system continues to exist.
