My idea was basically based on a seed (as usual in procedural worlds) that would be used to choose 2 million integers between 1 and 100 billion. These would be galaxy numbers.
One integer is coded in one byte, so the seed would take 2Mb. That's very expensive. You take 1Gb to emulate 500 galaxies...
I think you can need less resources (for example, a Minecraft world is based on a small sequence of chars), and you can find functions to generate more stuff with a "chaotic" aspect.
For example, take a random number x (between 0 and 1), and iterate x(n+1)=x(n)*k*(1-x(n)) : the comportement of the sequence x(n) will depend of k, and when k>3.5, it is very difficult to differenciate x(n) and a random sequence. Almost and all the RNGs (Random Number Generators) are based on such algorithms.