@BBeastHmm, If I had to say, Ull's mind imagery would be closely resembling the human nervous system - with a space background.
It would stretch and expand indefinitely, much like the Universe constantly expands, and fitting with Ull's theme of stellar evolution.
Countless nebulae would act as the "neurones" of the "brain." The "electrical signals" passing through the "synapses" would be sudden gamma ray bursts, constantly going off and lighting up parts of the "brain" like neurones do.
Should one dare tread inside Ull's mind, he would find himself disoriented at first. It would take some time to adjust to a 0 gravity setting unless the person has experienced it before (although this would be "spiritual" 0 gravity. Don't know if there would be a difference between the two.)
That's the rough draft. I am not gonna say anything else because some things might change and I want to be flexible with stuff like this...
EDIT: Sort of like dark matter, but more nebulae-y if you will..