My personal trick: as long as I'm confident that I will be able to refactor in the abstraction, I don't bother. I may or may not ever revisit that code.
It's bad enough that Google Drive/Docs notifications are treated as "external senders" so you have to open up your internal Google Groups to the world; then Google Groups flags them as spam anyway!
I'm guessing spammers may use Google Docs comments as a platform, but have never seen actual unsolicited Docs comments.
The usual configuration is 1 or many subnets per VPC, and one or many VPC per project. a Shared VPC setup between projects is also possible but requires prior agreement from both projects.
Dark Matter is just a placeholder name, don't read too much into the "Matter" part. There are no assumptions there and plenty of alternate explanations are proposed, like Modified Gravity (MOND)