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People born after 2000 have never seen the cosmic microwave background on their TV set.
(startrek.website)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
Maybe not directly on their TV set, but there are more than enough references to it in TV and film media that it's still known almost universally.
Everything from old beloved films to Modern period shows. Its literally an overused way to establish the narrative isnt taking place in the present.