Getter material evaporated and deposited on the glass. In physics sometimes a so called "titanium sublimation pump (TSP)" is used for ultra high vacuum systems. Titanium has a high "sticking coefficient" and gas just sticks to it. There's also ion getter pumps being used for just holding the pressure.
55.2V DC coming from the open loop boost converter. I still wonder how this can provide a stable voltage without any load attached. Theoretically the inductor should charge the capacitor more and more if it is constantly switched.
Adafruit recommends to wait a few minutes after testing the step-up converter before going on with the build process. This can be sped up by bleeding the capacitor with your finger across its pins _after unplugging_ the power supply. Skin surface resistance is low enough to make that work. This shouldn't be considered "best practices", but here it is safe enough. Using a 10k resistor to 'short' the capacitor works as well. Don't do this if you're poking around inside something with real high voltages (e.g. old TV sets with several kV).
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