When a block of coins
There aren't any blocks of coins. There are blocks of transactions.
coins is discovered
Coins are not discovered. Nonces are discovered.
discovered by an address
Addresses don't discover anything. Addresses are added to transactions as a way to create a requirement for a digital signature
the existance of those coins in the network
Coins don't exist in bitcoin at all. We humans use the concept of a coin as an abstraction to make it easier to discuss the transfer of control over value. There is no "existence of a bitcoin".
raises the difficulty to discover new coins
The difficulty that is adjusted is the difficulty to solve a new block. It has nothing at all to do with the "existence of bitcoins" and everything to do with the amount of time that it took to solve the the previous set of 2016 blocks.