The galaxy should be folded like this

The grid steps by 4 in one direction and 1 in the other
This gives a side length of $ \sqrt{4^2 + 1^2} = \sqrt{17}$

Pictures to follow soon...

The galaxy should be folded like this

The grid steps by 4 in one direction and 1 in the other
This gives a side length of $ \sqrt{4^2 + 1^2} = \sqrt{17}$

This is a crude model I made of the cube, with the joins highlighted (might have missed one).

First of all I spotted the 4 quarter-grid parts, which I assumed (and was right) would be the centre of the opposite face. After a few false starts the penny suddenly dropped as to where the $17$ area comes from, and from there I produced a model.

OP previously said he would make these puzzles harder, and it was. Unlike others, there is no obvious place where the corners of the top side will be. The centre part does not include any of the corners of that face.