All these Geoms I’ve been collecting – hundreds of thousands of them – were all used up by the last few planets in the penultimate galaxy. Meaning I didn’t have enough to open the final galaxy. So I went back to some earlier levels and managed Gold where previously, I’d only managed Bronze. That let me in, and with the first three planets there all Gold’ed, I’m back in huge credit!
Sadly, it costs LOADS to unlock each planet, so it looks like I’ll need to get golds on them all in order to continue. Bah!