Yes, you can see every single possible address. These are visible because nothing an address does requires it to be "registered" onto the blockchain. An address is a special encoding of the RIPEMD160 hash of a SHA256 hash. To computers, these hashes are simply really big integers. Because we know the size of a RIPEMD160 hash (160 bits), we know what the biggest number that it can represent. This happens to be 2^160. Thus every address is already known because we know what all of the numbers between 0 and 2^160 are and can thus get the address without ever knowing the private key.