When I asked my neurologist what the nonspecific brain disease was, he said "Oh it's just fat or cholesterol deposits." I don't have confidence in his abilities to diagnose me. I am definitely getting a second opinion.

You need to see another neurologist for a second opinion. The radiologist knows what fat and cholesterol deposits look like and would not call it a "disease". The radiologist would have noted their appearance and said something like "consistent with this age group" or "appearance normal". You need to go to a neuro that will look at your actual scans and not just the report and see what it is that the radiologist is calling a "disease". I suspect that prior to the overall impression, in the report there is also something about small white matter hyperintensities in axial flair imaging...probably in the periventricular white matter or corpus callosum. If so, that is what the radiologist is calling "stable white matter disease". Generally, they would call it stable if it didn't show changes when the contrast was added or if the numbers didn't change from a previous scan.