Procedural generation and permutations question

I probably won't pursue this but I had this idea of generating a procedural universe in the most memory efficient way possible.

Like in the game Elite, you could use a random number generator based on a seed, and so each star system can be represented by a single seed number instead of lists of stats and other info. But if each star system is a 64 bit number, to make the milky way, 100 billion stars, that is 6.4 terabytes of memory. But if you use only 8-bits per star system, you'll only have 256 unique star systems in your game. So my other idea was to have each star system represented by 8 bits, but simply grab the next 7 star system's bytes in memory and use that combination to form a 64 bit number for the planet's seed. Obviously there would be 7 extra bytes at the end to account for the last star system in memory.

So is there any way to organize the values in these bytes such that every set of 8 bytes over the entire file covers all 64 bit values (hypothetically) with no repeats? Or is it impossible and I should just accept repeats? Or could I possibly use the address of the byte itself as part of the seed? So how would that work in C? Like if I have a file of 100 billion bytes, does that actually take up exactly 100 billion bytes in memory or is it more and how are the addresses for those bytes stored? And is accessing large files like that (like 100gb+) in a server client relationship practical? Thank you.